Animals do this shit all the time, I think it's not that bad honestly. I mean, she seems hot enough, and she's always near by when he's in the mood. The Japanese masturbate to incest-written porn scripts, I don't think it's as wrong as people say.

Then again I am an only child and maybe if I had a sister I'd probably be in the boat with you going "ew ew ew ew ewwwwwwwwwww".