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Katsuki has always been the one to make their family a home for his bother. Their parents are gone, constantly- Katsuki thinks that they have only been to two of his birthdays for his entire life. They assume their money will do the loving for them.
So, as the older brother and as the Omega, he feels it is his duty to make sure that Eijirou has the best life there is.
It helps that he adores his brother so much. Eijirou is a cute kid, with chubby cheeks and lush red hair and so much energy, it can be hard to keep up with him. And when his claiming fangs came in, it became clear that he was an Alpha, so Katsuki felt even more protective about him.
So he takes care of Eijirou, considers it his one goal in life to make sure Eijirou turns out a healthy and proud Alpha for society.
Maybe that’s why he doesn’t stop Eijirou when the lines start to blur.
He’s laying on the couch one day, watching tv after a hard day of schoolwork and athletics and he just wants to relax. Then he hears the pitter-patter of feet coming to run. He has a moment to brace himself before Eijirou jumps on top of him.
“KittyKat,” Eijirou giggles. “You’re home.”
Katsuki smiles even as he helps Eijirou to get comfortable on him. “Hey there, Ei, did you have a good time today?”
Ei pouts at him. “It was so boring without you here. I missed you.”
“You say that every day.”
“Because it’s true every day.”
Eijirou starts to squirm, the frown deepening. Katsuki notices, because he always notices. He wipes a bit of hair from Katsuki’s forehead. “What’s going on with you?”
Eijioru huffs. “I keep chewing on stuff and the maids keep telling me I shouldn’t. But the itch is too strong.”
Katsuki, of course, knows what’s going on. Eijirou is an Alpha pup. Alpha pups start getting certain urges early on. One of those urges is to help their future Omega lactate to help with pups. Usually, there are two ways to help with this. Either a.) there are plenty of toys that help with these urges that he can get. Or b.) the pup’s damn can help by letting them practice those urges on them.
Katsuki hates the thought of letting Eijirou suffer any more while they wait to get a toy. And since their mom isn’t here anyways, well. It’s up to him to take that role.
“I have something to help you,” Katuski tells him. He lifts his shirt, showing off two dusky nipples sat atop of his perky chest. A real Omega chest, other Alphas his age have told him with disgusting leers.
But that doesn’t matter. What matters is Eijirou. Eijirou who is tilting his head, confused. Confused till Katsuki gently grabs the back of his head and brings it down. “Go on, Ei,” Katuski coos. “You can suck on these. It’ll help, I promise.”
Eijirou’s eyes flick up to Katsuki’s face, then down to his nipples. He wraps his mouth around one and hesitantly sucks.
His eyes light up, his Alpha brain finally settling down now that he’s doing what it’s been telling him to do.
Katsuki leans back on the couch, going back to watching tv as Eijirou suckles on him, going back and forth between each nipple. He tries to focus on what he’s watching and not how good it actually feels. He’s never had an Alpha touch him like this before. Never felt safe with any other Alpha.
But Eijirou is his little brother. Not only is he safe with him, but he would do it anyway, even if it felt gross.
Anything for his brother.
~
That had been the first instance for them. Just a brother wanting to help another.
The suckling continues every day. Katsuki gets home from school, Eijirou runs to him, and he sucks on Katsuki’s nipples as they watch tv till one of the maids come to get them for dinner.
While Eijirou had started this hesitant, just sticking to lightly suckling on them, he soon grows more bold. He licks at them, gets Katsuki’s nipples all nice and puffy and red. Even scraps his teeth across them. When Katsuki gasps at that, he does it again and again.
He doesn’t stay to licking either. He gropes at Katsuki’s chest, little fingers twisting one nipple while he suckles on the other, before switching them. He squeezes Katsuki’s plump chest and plays with them, bouncing them around. He even motorboats them occassionally.
Eijirou seems to have a large fascination with Katsuki’s chest after that. Katsuki finds Eijirou sticking his hand down his shirts when they sit together to do homework, giving one pec a squeeze. Or when they hug, little greedy fingers find their way to his nipples.
Katsuki doesn’t think much of it, not really. As Alpha grows older, they get more perverted towards Omegas. Of course Eijirou is going to cop a few touches here and there.
Then his brother starts getting even more bold. Bold enough that Katsuki finds a few of his own underwear in Eijirou’s room, as well as seeing little eyes peeking at him through the crack of the door as the Omega undresses to shower.
A small part of him thinks he should put a stop to it, should sit Eijirou down and explain that he shouldn’t be treating Omegas like that, and he really shouldn’t be looking at his brother that way.
A larger part though…a larger part just thinks it’s too cute of him. The fact that Eijirou is doing such a bad job of sneaking around to be a perv to his brother is adorable. That, whenever Katsuki catches him watching him, the little Alpha loudly runs away. That he can’t help the blush on his cheeks or how he squirms in his chair, pulling his shirt down over his lap.
Eijirou is just so bad at being a perv and his reactions to being caught are too precious that Katsuki can’t bring himself to stop him.
So things continue to spiral.
~
Katsuki has to admit, he didn’t see the next part coming.
It had been a tiring day. Practice had been hard, the homework had been long, and all the energy he had to do was eat and go to bed, though he did make sure to give Eijirou some of his time, to make sure he knew that his big brother was proud of him.
Then he went right to sleep.
Except Katsuki has never been a heavy sleeper. It doesn’t take long for him to feel a weight on his chest. Of course, he keeps his eyes closed. Most likely, it’s just Eijirou.
Proven when his shirt is pushed up. Eijirou must be feeling the aches again, his gums itching. He just needs to release them a little. Katsuki is more than ok allowing that to happen. Eijirou will be ok and Katsuki can get some sleep.
So he settles back to bed. Except Eijirou doesn’t lean down to lick at his nipples or even grope them. He does nothing more, than let Katsuki’s tits shiver in the cold, his nipples pebbling up.
But while he doesn’t grope Katsuki, he does make these odd little noises. And the bed starts shaking.
Katsuki has to look. Because it involves Eijirou and he has to make sure his little brother is alright. So he peeks open his eyes and has to stop his body from jerking.
Because Eijirou is jerking himself off on top of Katsuki.
Like, full on has his dick hanging out of his pants, one hand wrapped around his cock while the other braces himself over Katsuki. Katsuki is absolutely dumbfounded, both by the look in Eijirou’s eyes, as he leans over them with his tongue sticking out, and his dick.
Katsuki has never seen an Alpha dick before. He knows they’re supposed to be big and thick. But he had no idea that they would be so…veiny. There’s a thick one, throbbing right near the tip.
It’s an interesting thing to observe. Even though Eijirou is still young, his cock is so big. And so red, like it’s angry looking. It makes him wonder what it would look like when Eijirou is older.
Again, that small part of Katsuki speaks up. That he should push Eijirou off and punish him in some way, let him know that nothing about this is ok. Let him know this is wrong and to never, ever do this again.
But that larger part once again has him just laying back and letting Eijirou get away with it. After all, he’s a young Alpha, going through puberty, living with an Omega. Of course he would be curious and needy.
Most likely, this will be the only time that Eijirou does this. Might as let him get it over with.
After all, he’s so cute, with his flushed face and desperate huffing.
~
It is not the last time.
~
Katsuki doesn’t catch Eijirou every time he does it, but he knows that his brother is still masturbating to him because…well…Eijirou isn’t that good at cleaning up after himself.
And Katsuki continues to let him do it. Eijirou doesn’t know any better, Eijirou is just exploring his Alpha side, Eijirou will calm down when his hormones do.
And as the big brother, it’s Katsuki’s job to let him do that, right?
So he sees Eijirou sniffing one of his panties while he uses another to jerk off? That’s totally fine, Eijirou will get over it. So he see’s that Eijirou won’t leave the room when Katsuki gets undressed? That’s totally fine, Eijirou will get over it. So he see’s that Eijirou finds more and more excuses to grope his tits and for Katsuki to bend down and give him kisses? That’s fine, Eijirou will get over it.
Katsuki let’s it all go, let’s his baby brother continue his perversions, so sure that he’ll stop it one day.
Till the time comes that he can't.
~
Katsuki hasn’t seen his brother all day, which is weird considering Eijirou is usually glued to his side.
At first, he hadn’t thought all that much about it. He’s had to have his monthly phone call with his parent, to let them know everything is going fine and that his grades are where they should be, and a reminder for him to start applying for universities.
Then he had to have Archy lessons with his teacher, who had been rough on him as usual. After showering though, he realized that Eijirou hadn’t been secretly watching him like usual.
Worried, he goes searching for him.
While Eijirou and Katsuki have their own separate wings of the house, Eijirou likes to sleep in the room next to Katsuki’s- or crawl into bed with him to pass out after he cums across Katsuki’s chest. So when Katsuki goes looking for Eijirou, he of course, goes to his own bedroom first. Except Eijirou is also not there.
But it looks like his room had been ransacked.
Clothes are spread out across the floor, his bedsheets had been ripped from the bed, like someone had just grabbed as much as they could before leaving.
It was baffling, and Katsuki could only stare dumbfounded at it for a few seconds. Then, his nose starts working again, twitching as a particular scent hits him.
It’s Eijirou’s scent, but deeper. Richer. There’s a hint of something nearly feral in it.
It has Katsuki’s heart thumping harder. Something is wrong with Eijirou. Something is wrong with his baby brother.
Katsuki backs out of his room and runs towards Eijirou’s, following the scent. His Omega is howling at him. He needs to find his brother, he needs to find Eijirou, he needs to find his Alpha-
He throws Eijirou’s door open. And is hit with a blast of rough Alpha stink. It nearly bowls him over, so strong as it is.
Looking through watery eyes, he sees Eijirou crouched on the ground, making little whimpering sounds. If Katsuki looks closer, he sees that what Eijirou is humping are his lost clothes.
Eijirou’s head snaps up, his mouth being pulled open to show large Alpha fangs. His eyes are dilated, blown out. He’s sweating, hard. And his cock looks like it’s going to explode at any second.
“K-Katsuki,” he whines and Katsuki’s heart plummets to his stomach. Poor Eijirou is in rut. His very first one.
Ignoring every warning he’s ever heard, Katsuki walks into the little Alpha’s den, kneeling down next to him. “Eijirou,” he whispers. “What do you need?”
“I need…I need…”
“Name it, I’ll help you brother.”
“I need… you .”
Katsuki yelps as he’s pushed onto the ground, his little brother crawling over him, still humping away. Now though, his wet cock slides against Katsuki’s leg, smearing precum everywhere.
“E-Eijirou, you can’t-we can’t-you need a rut aid,” Katsuki tries, finally pushing Eijirou off.
Only, Eijirou won’t go.
The Alpha is too strong now and really, Katsuki is too weak for his brother. Always has been.
Especially when Eijirou looks at him with watery eyes, tears trailing down his chin, and lower lip wobbling. “Please, big brother. I need it, please.”
Katsuki is the older brother and he is an Omega. It is his job to help his younger brother out.
So. He helps his younger brother out.
~
“Ah, ah, ah,” Katsuki whines. He wishes he could reach down, push is brother away, but that wouldn’t be a good big brother thing to do.
So he lets Eijirou continue to eat him out.
When Katsuki had agreed to help Eijirou out, he thought he would just jerk the little Alpha off till his rut passed. Maybe let him suck on his tits again.
That was not what Eijirou wanted though.
He shoved Katsuki to the ground and ripped his clothes off, before spreading Katsuki’s legs open.
Spreading him open for his pussy to be revealed to hungry eyes. Katsuki had gone to cover himself but Eijirou had nipped at his hands. Then he had dived down and started licking Katsuki’s pussy, over and over again.
Katsuki shivers, stuck between horror and pleasure. He’s just helping Eijirou out…but this is his brother, touching him where only his mate should touch him. Only a mate should lick at his chubby lips and suck on his clit and drink down his slick. Only a mate should dig their nails into his thighs to spread him open even more.
Only a mate should be giving him the heated looks Eijirou is, desperate for more.
And it’s only a mate that Katsuki should cum for, squirting more slick onto Eijirou’s face, who quickly licks that up too. And still Eijirou continues on.
It’s confusing, because only a mate should do this. But Eijirou isn’t a mate, but a brother…and yet, as his hips roll up into Eijirou’s tongue, his Omega brain can’t tell the difference. Mate? Brother? They’re one in the same. One Alpha he feels safe with. One Alpha that can take advantage of him.
One Alpha. One mate. He squirts again and shouts “Eijirou.”
Eijirou’s eyes glint gold. He finally lets go of Katsuki, but that’s only to flip Katsuki onto his hands and knees.
The little Alpha climbs up Katsuki’s body, panting against Katsuki’s back. His large dick squeezes between Katsuki’s ass. He ruts against the Omega, whines and whimpers dripping out of his mouth.
“Omega, ah ah, Omega, can I? Can I? Please, I need it, please-”
Eijirou is begging for him like an Alpha begging for his Omega. Begging for his big brother to help once again.
So Katsuki reaches back, grabbing that Alpha dick. It pulses in his hand and Eijirou whines, humping into the flesh. Katsuki could stop it there, could just milk Eijirou till he’s done.
Instead, he lines Eijirou up with his cunt and helps the Alpha slip inside.
Once Eijirou feels those warm walls around him, milking him for all he’s worth, there is no stopping him. He’s too young, too uncoordinated. He doesn’t know any better.
Which means he immediately begins to hump away, pounding into Katsuki as he holds onto the Omega’s waist for dear life. The entire time, he thanks Katsuki for allowing it. “Thank you, Big Brother, thank you thank you thank you-”
Katsuki’s hands dig into the floor, every bit of air pounded out of him. “Oh god, oh god,” he whines. It hurts. It feels good. He doesn’t know what to think. He shifts his hips, just a little, and it hits something pleasurable in him.
Katsuki is suddenly seeing heart eyes with every thrust into him. His tongue rolls out. “Eijirou, Alpha-”
“Omega, Omega, my Omega.” Eijirou’s growl deepens. It’s the only warning Katsuki gets, before Eijirou bites him.
The pup can only reach up to his upper back. Katsuki moans, pussy squeezing Eijirou’s cock. Which just makes Eijirou bite him more, just makes the Alpha fuck Katsuki more.
Katsuki can only hold on as he’s fucked, slick dripping to the ground.
It doesn’t take long for Eijirou’s knot to expand, considering how inexperienced he is. It leaves Katsuki wanting, leaves Katsuki with the realization that he’ll have to teach Eijirou how to be a good Alpha.
And as young Alpha cum fills up his womb, he giggles at the realization that he’ll have to teach Eijirou what it means to have an Omega, what to it means to have to mate, and how to satisfy both.
Like a good older brother should.