Not trying to add more to the ntr thing, yes she is his girlfriend.
In this case, it is debatable, not only does it look like he is cheating but it seems that he doesn't plan to stop with the sister. He said something cryptic that could mean that he plans to go after her friend who can be another love interest
I think he will serve as an antagonist who will take the girls if we make bad decisions.And he may not be alone I mean we meet the mom when she taught she was seeing an escort
I hope it is just optional or at least they make sharing/swinging it not avoidable, just not netorare
I haven't played the Unity version so I may be wrong. I'm still interested in the game and I'll stay to see where it goes from here.
Edit: Dev post when I was writing and now I feel stupid