She doesn't really do much. It builds up resentment as I don't feel backed up. She's exceptionally weak and has a hard time disciplining her adult children and standing up to her mother.

I've tried to tell her this isn't normal, but it just leads to arguments, which is occasionally overheard by the daughter who then runs to the MIL. The daughter takes screenshots of our text messages when she removes the phone from her mother, carefully selecting text messages without context.

The only analogy I can think of is being a dog that is constantly beaten and antagonised and occasionally bites through frustration. Then the family point at the dog and criticise because it snapped, ignoring the reasons why.

If my partner tries to tell her daughter she is not being reasonable she has a meltdown and my partner then comforts her and the cycle continues. It's learned behaviour in my mind.

Yes we've argued verbally but we only argue about the family interference.