From what I was told, there was a significant legal fight over sexual abuse allegations made by several other (female) family members. Apparently it came down to "He said / she said" and the charges were dropped.
She tried to adjust the custody arrangement to keep my father in law from my W and SIL and sent both kids to counseling. None of the allegations were corroborated and the custody arrangement was not adjustment.
My FIL's family was essentially disowned him and he has not had contact with them for years. When we had kids, my MIL insisted that we did not let FIl watch our kids.
My W insists that it was all nonsense and that she was never abused. I tended to believe her thinking "Why would she lie", until I had the conversation with her the other day where it was clear that she had completely suppressed the reality of us having sex in the last year. Her denials were completely sincere.
