If NTR is avoidable? Just like a route you just skip.
I believe the walkthrough removes the option that leads to it. But if you don't use it, then just choose to protect your stepdaughter and it won't happen. It isn't shown, I think, it's only implied even if it does happen. 100% avoidable, though, as I didn't see it. If you're super against other guys touching your girls, there are a couple uncomfy scenes but nothing really too bad.
I hate... that acronym... and I am a very jealous person and this game is honestly fine. Your stepdaughter is going to experiment with her (female) friends. But it's not like you can't also have your fun with them, too.
The talk about it is MUCH worse than what actually happens here. Even the G/G scenes aren't really there to cause jealousy. As I said, your stepdaughter is exploring this
brand new world of sex. Pretty sure it was mentioned that you will also be the only guy she is with, at least without your say.