alban, young and precocious, wakes up from a bad dream and goes to find sonny for some comfort. unfortunately for them both, sonny is in the middle of masturbation. sonny, ever trying to be the good onii, grits his teeth and puts a pillow over his lap and tries to calm himself down because his little brother needs him.
alban doesn't know yet why the sight of sonny's flushed face makes him feel... hot and squirmy inside, and sonny's just trying to hold shit together when he was so close before he was interrupted... alban can feel that he's hard underneath the pillow while sonny's reassuring him and he's naturally curious, so he asks what's up, and at first sonny's trying to deny that anything's different, but alban looks at him with big eyes and goes "oniiii" so he's forced to reveal himself, still hard, and try to explain the human body to his younger brother- and he flinches because alban, innocent, reaches out to touch and interact with his erection, surprised by how hard it is, how big it is (will mine do that?), that liquid comes out (is it pee?)
sonny's trying so hard to be normal about this but fuckkkk he's so horny and something about the debauchery in all this, how fucked up it is, he's only getting harder... alban asks how he gets his to get big like that and sonny's too far gone to explain, offers to show him instead- might as well teach him early before he can make the kind of mistakes sonny did, right? make it... make it good for him...?
he pulls alban into his lap and helps him work his pants down, slicks his hand with his own precum (and fuck that's so sick, it's so wrong, but god something about this is so hot...!) and starts touching alban- shushes him, tells him to try to keep quiet when he starts reacting to the overwhelming sensation of being touched like this for the first time ever. he puts alban's hand on his little cock and teaches him how to make it feel good, tells him it's ok and it's normal and all the while alban squirms against him and sonny's still trying to convince himself he's just being a good brother looking out for his precious otouto, and that he's not getting off on the feeling of alban's smooth skin against his cock.
unfortunately alban's precocious, and he turns around to look at his precious onii because he feels all hot and squirmy and in doing so he brushes his little prick against sonny's own erection, and that feels even better than hands, so he asks if they can do that instead- "wanna try it, feels real good", and how can sonny ever tell alban no? he's too far gone to possibly reign this in now. sonny leans down and captures alban's mouth in a kiss as he frots against him, and alban tries to mimic whatever he's doing because it feels so good- he gasps out that he feels funny and like something's going to come out of him and sonny rubs his back, tells him to let it happen, soothes him through the first orgasm of his young life- and the broken way alban chokes out "onii-chan" is what causes sonny to break and spill all over alban's little cock, making a mess of his precious little brother while the poor guy is still pleasure-drunk from his first orgasm.
sonny, more aroused than he's ever been in his life, immediately gets on his hands and knees and starts cleaning up the evidence- any other time he'd be completely turned off from eating his own come, but mixed with the clear fluid alban's released, tasting the salt on his flushed skin and leaving him giggly from the way it tickles, is the hottest thing he's ever done. it's fucked up and wrong and the shame eats him up inside for weeks after, even though it's the memories that bring him to release time and time again.
after the fact alban is wore out from all the new experiences. sonny tells him, gentle but secretly nervous, that he shouldn't tell anyone else about what they've done. they fall asleep snuggled up, the picture of innocence, and sonny promises himself that this can never happen again.
in the intervening years, naturally, sonny and alban exchange all their firsts and wind up having a "great relationship as brothers" that's definitely parasocial and codependent but sonny can't let anyone else steal away his precious otouto...
and alban's not going to let anyone else take what's his.
sonny trying to justify this to himself, e's just making sure alban doesn't make the same mistakes he did... he never tries to do anything again, though.
it's alban that starts pushing it. "make me feel good again, onii... i wanna feel good again..." alban's little mouth kissing him clumsily makes sonny growl and grab his pert little ass, kiss him like he's old enough to know what they're doing, leave him trembling and dazed and overwhelmed as he kisses his way down alban's little body and pulls down his pyjama bottoms with his teeth. so gentle, so caring, he's gonna be the best brother and make alban feel so good, so good that it ruins him for anyone else, so that only sonny can satisfy him.
and once that door's open, alban's just as bad. he's a fast fucking learner, especially when he's motivated, when he wants something -- and he wants sonny.
as he gets older, realizes what sonny did to him, there's no resentment- far from it. he's already having the best sex of his life when most of his classmates haven't even held a hand. there's nothing sonny won't do for him, anything alban wants to try, any experience he wants to have, sonny will give it to him. even teaches him to do it better, make it good for both of them... the first time he fucks sonny, sees him spread out beneath him, skin flushed and mouth open in a perfect gasp of pleasure, he realizes how fucked up this all is, it really sinks home for him how fucked up it is to be fucking his brother like a dog in heat- but it's not a deterrent. how could it be? he fucks into sonny like he's carving out a home for himself and leaves a mess behind, whispers absolutely filthy invectives that even sonny's startled to hear- "mine, no one else can have you like this onii, i won't share you, you're mine!"
alban and sonny have an addiction in the shape of each other- and against all the odds, they never get caught. never grow out of it. they can't stop now, this is all they know- even when they start including other people, it's not because they aren't fulfilled, it's because seeing each other work someone else open, ride someone, seeing pleasure on their face is just as hot mentally as physically- and their friends aren't stupid, it's impossible to miss the way alban and sonny are around each other.
uki's probly the first one that brings it up i figure. man reads people well even if he wasn't psychic. naturally if uki's aware, fulgur's gonna get in on it... i feel like he'd have reservations, especially if he learns how fucking early sonny and alban started fucking around, but... consenting adults................. and both people he cares about and probably knows well, too...
i feel like alban's gonna come onto fulgur, he's the greedy one. uki doesn't like to share much either so he's going to get sonny out of half spite, half drawing everyone into something... in the end they're going to have a relatively stable polycule.