There's a fatal flaw in this premise.
When she gets horny, her horns come out.
The more horny she gets, the bigger the horns get.
The bigger the horns get, the stronger and more dangerous she becomes... because her strength gets more and more uncontrollable as it keeps increasing.
Oni kouhai will never be able to make love to her senpai without accidentally killing her. >_<
Obviously the solution is she's gotta be the bottom.
worst imaginable scratches incoming for senpai's back, and let's hope kouhai isn't a biter >_<
more seriously it's a really well executed story, super cute, well paced
and the, well, the mechanics of a relationship have to have worked out previously for that family tree to have not ended. the horned ancestors have non-horned partners so there's gotta be some trick to it.