NAKAIMO - My Little Sister Is Among Them!

Synopsis

Is there a genetic predisposition to insanity? Shougo Mikadono is starting to wonder, because the conditions of his late father's will seem completely absurd—and trying to follow them might drive him crazy too. The premise sounds straightforward enough: before Shougo can claim his very substantial inheritance, he simply has to enroll at a specific new school and find a suitable girl to marry. A bit unromantic, but certainly doable, right? After all, the girls appear friendly enough, so all Shougo needs is to find one he shares something in common with. EXCEPT—and here's the catch—it turns out he shares way too much in common with one of them, because that girl is actually his long-lost sister! And he has no clue which one she is. Can Shougo meet and woo his future wife without committing something deeply immoral? Will he manage to find his bride without veering into the wrong gene pool? And if his little sister does come forward, just how much will be exposed, and under what circumstances?