Namechanged for obvious reasons.
My BIL has always been a bit, well, leery. It got to the point where I would avoid being around him when he was drinking as he would put his hand on my knee, get too close whilst dancing etc.
Every now and then he would facebook something very flirty (is this the sexchat service etc) and I would ignore it.
He recently turned up a my house on the way home from work but luckily I happened to notice his car parking out the window so hid in the back room pretending to be out. I know he had no reason to visit and refuse to be alone with him. I turn down lifts etc
I have always sated I'm not interested but he thinks it's a joke and carries on.
His relationship with my sister has been up and down a times. She was told via friends he had a reputation for being a "dirty old man" and laughed it off. She fully backed him when another person disclosed an incident and walked away from people over it. She is very stubborn about it and I just felt I didn't want to risk our relationship over it. There has been moments when she has admitted she believes all hat is said but that she could turn a blind eye if it wasn't actual sex. (I would bet my house on him having cheated on her fully btw)
The problem now is they have separated. It was instigated by her as she is struggling with depression and unhappy with the marriage. But she is not stating any of the above reasons.
I am obviously biased and think she should stay very separated from him and I think knowing what he has been like over the years may help her make a more informed decision.
On the other hand, they have split and rekindled a few times and if I tell her it will mean the end of us should they get together again. There is a very high chance I would lose my sister and her children over this which would be unbearable.
AIBU in keeping this from her?
I honestly can't decide what to do about it.