Flowers for your girls OP, and for everyone on this thread who have suffered at the hands of abusers, or who is supporting those who have suffered.
IScorched earth as Kaleela* says is exactly the right policy here. Total NC with MIL (and any flying monkeys) with immediate effect. No contact in person, by phone or email and block all social media. No negotiation, listen to no pleas, make no exceptions.
As you already know, you are right to back your girls absolutely.
You have to get clear of that absolute chess-pits of attitudes and actions or it may well end up consuming another generation (you also know that already).
When I was younger, I was friends with someone who had been abused as a child by a family member, from a very young age into her teens. She had lots of mental health/emotional/sexual identity issues and was very messed up.
When she told her parents about the abuse, her mother alternated between dismissal/denial and telling her to basically put up and shut up because the relative was financially advantaged, connected and useful to them. Even when abuse of other similar aged people in the family emerged.
The family “solution” was to get the abuser to pay for therapy/private hospital stays during the (regular, probably most years) mental breakdowns their daughter had from early teens onwards. She was heavily, heavily medicated from her early teens. Kept trying to get her mother to believe her, to accept her, to show her some love and compassion. She went to a lot of family therapy sessions to reconcile with her mother, which were just extended torture for her really. She remained in contact with her family and financially dependent on them to an extent, although at a bit of a distance because she wouldn’t fully “move on”.
When she was in her early thirties, it emerged that the next generation in the family were now the targets of her abuser, as well as some of her male cousins, her contemporaries, some a few years older, some a few years younger.
Those cousins had been abused around the same time she was, both directly by the abuser and by being forced by the abuser to “perform” with her and one another for his benefit. She had an extended, intermittent sexual relationship with one of these cousins, let;s call him X, who was older, from when she was ten and he was fourteen until they were both in their twenties. They would meet up in secret (without the knowledge of the abuser or their parents ) to have sex. This continued after both sets of parents learned both of the abuse and the relationship and tried to keep them apart (as it was “dirty”). She described the relationship as “loving, healing and therapeutic”.
The next generation abuse emerged when a younger female relative who was being abused approached her for support and help (as she was known in the family for not totally towing the line). This young girl was being abused both by the original abuser and the cousins, including the one who’d had a long relationship with my friend.
My friend’s response was to tell her young relative that she didn’t believe the abuse was happening, that she didn’t think X especially was the kind of person who would rape someone, that family connections were useful, that she herself had benefited greatly from connections to the people involved (property advice and help with house deposit) so the best thing to do was to keep quiet and not rock the boat. In short, she just repeated her mother’s behaviour.
Luckily the young relative did not leave it there and all hell broke loose.
What I’m trying to say is that in this kind of family, when there are many levels of inter generational abuse, total repudiation of abusers and support for victims is the only way the cycle gets broken, the only way that those abused are able to heal.
I cut ties with my friend when I learned what she’d done. She’d cried to me so many times about how she had suffered, both from the abuse and from her mother’s reaction. And then no sympathy or even belief for the younger relative. I know people often struggle to have insight into their own behaviour, but it just seemed so dangerous.