Meh, i prefer this one.
It isn't, it wasn't right of her to take his virginity and not even let him enjoy it, and then continue raping him in his sleep without letting him have fun as well.
She's a very selfish imouto who doesn't care about her onii-chan enough.
Yes, it's hard to get anymore pure than a yandere imouto.
The imouto aspect literally does away with any concerns about the intensity of a yanderes affections because her onichan has been a central part of her entire life.