I would add that this story and some others make more sense without incest.
I'm not an expert at all ( ofc), but for me, to go towards incest needs one of two things.
1. It's pure manipulation, usually more by an adult on a teen ( or worst, a child), to not be obvious and all it needs time.
2. It's mutual and involve strong feelings before becoming physical and go through the taboo barrier, and it needs time.
If you want your story to have a nice pace and things to happen, it makes more sense to not have the taboo barrier here in the first place.
Here, the two girls had a friend when they where young and probably had a crush on him. Now he's back, not bad looking and not an absolute moron, the feelings are back then go all out.
For Monica, frustation for some years, a young man she has known to be nice and all come back, again not bad looking and treating her well, let's open up.
I had nothing against incest, some of the games have it and they're really good. I just don't see it as a must have.