Voyeurism. One scene with some prostitute and the other one with Lady Hale. It doesn't really count as NTR since the MC does not have any romantic feelings towards them, but it definetly count as cheating (not towards the MC though).
anyone can tell me what happened to the sister?
If you mean after the ship crash, well, it's a mystery. It seems she disappeared in the forest.
Buut if you mean about what happened with her and the uncle... It is implied that she was abused. Though I hope he just experimented with some crazy shit and that's why she's fucked up in the head because... Well it's kinda of a turn off for me to see that stuff in a hentai game (it's at least handle serious though, and it manages to make me feel kinda pity for the poor sister).