<human>: Give me the gist of the text in a nutshell

The title pretty much sums it up. I found out only because she made a Facebook post about it tonight. I talked to my dad about it and apparently this has happened before with others. 

They live in a small town where pretty much everyone knows everyone. My cousin is from my mom's side of the family. My step mom also used to date my other cousin who is the stepbrother to the one she cheated with.

My father says everyone expects him to be fine since he's supposedly an asshole, which he isn't. He doesn't express love very openly, but you know when he cares. I read comments on my stepmother's post and all of her friends act like it's her fault. I don't know what to do. 

My cousin spends a lot of time at their house, which I was uncomfortable with to begin with. He may have sexually abused me when I was 4 and he was 15, but he has an identical twin brother he is close with, so I will never know for sure it seems. 

I'll also mention I've never lived with my father, and our relationship has been rocky because he wasn't around as much as he could be. I want to suggest that he divorces her because I'm pretty sure the only reason she's around is to watch my siblings (F13, F10, M10). She sits around and watches television all day while he works, shops, cooks, etc. 

My father doesn't know about the sexual abuse either. I'm not sure if I tell him about that either.
<bot>: My stepmom cheated on my father with my cousin who sexually abused me. I'm not sure whether or not to tell him about that and also whether to suggest he leave his wife.