The saddest thing I witnessed recently was the aftermath of a situation very similar to the OPs. Family member who is an adult SD in her mid 40s treating the SM appallingly, targeted bullying the likes of which I have never seen before. There was an agenda, she needed them to split up.
The DF, not wanting to take sides, stuck his head in the sand and hoped it would all work out eventually. This went on for several years until the SD used the ultimate weapon and has been cut off.
They are ALL to blame, the SD for being a despicable, selfish human being, the DF for being a weak willed coward who stood by and watched as his wife was being repeatedly abused and the SM who kept going back for more, time after time and allowed the SD to treat her like shit for years with not a single boundary in place, waiting for the DH to erect them for her.
The SD is now persona non grata, suffering from severe depression and anxiety and feels like she has "fallen into a pit of despair", I can't help but wonder if she passed her SM coming out as she was falling in.
She firmly believed that anything she done to the SM would not affect her relationship with her DF - because "she is his daughter and should always come first"!
Although not directly involved in any of it, my life (and the lives of others I love) has been negatively affected by the whole thing and I am mad at all three of them.
People need to pull their heads out of their asses if they think an adult "child" can somehow be excused for shit behaviour on the basis of the relationship.