As much as I love Dadbastian, I'm also fond of the idea of Big brother!Seb!
Whether in canon or AUs, Brobastian is certainly a valid concept! He may be grown, but that sure as hell doesn’t mean he knows what he’s doing. He hasn’t even had a romantic partner in centuries, much less raised a child on his own...and now he’s supposed to look after some bratty kid?! So much for late-night parties and the exuberant freedom of youth; now, Sebastian has to worry about putting O!Ciel to bed on time and figuring out how to prepared balanced, nutritional meals. The two constantly drive each other crazy, undermine each other, bicker over the slightest difference in opinion. Either of them would kill you if you sought to harm his brother. Sebastian snickers when O!Ciel commits a faux pas at an important society dinner, and never misses an opportunity to poke fun at how short the puny human is. He gently bandages the younger boy’s knee when he falls and scrapes it, and quietly hugs O!Ciel after he wakes up screaming from yet another nightmare. Sebastian wishes that a wiser, adulter adult, a person who actually understood how to tend to mortal children, could take charge. He’ll always be there for his little brother, however loudly he protests that he doesn’t care.
