My grandmother was a lot like the MIL that pinkpuppy describes.
She wore black to my mother's first wedding. Yes, like a funeral. She sat in the front row and openly - and loudly - wept for the entire ceremony. She later "apologized" to my mother for the scene right before my mother had a very painful bladder surgery, offered to care for my mother after said surgery, and then withheld pain medication and bathroom privileges from her for nearly a day to punish her for whatever slight my grandmother had made up in her head in the first place. This was years after the initial wedding (and, in fact, I believe after my mother had already divorced that particular husband).
I get drawing a line in the sand, but I think it's also important to remember that beyond trying to keep the peace in the family in general, that these types of people fight dirty, they fight often, and they almost certainly are better at fighting than (generic) you are. It's one thing to think in your head "Okay, so they might be mad at me for a few years, their loss!" but it's quite another to experience it full force. My sister can be like this and she once told lies about me to everyone she knew and got them to send me hate messages on facebook telling me to hurt myself when I was 14 years old. Sometimes the preoccupation isn't with you hurting their feelings, it's with you challenging their psychological power and it will not be tolerated in many cases and they will drag anybody you know into it if they feel like it will work to demonstrate power over you. It may be THEIR problem, but they will make it YOUR problem over and over and over again.
Stepping away from these types of people is hard and it takes a lot of time and effort and, honestly, some convincing of the family members who they never seemed to treat poorly in the first place and who think you're lying about all of it. Depending on how bad they are sometimes stepping away from their behavior is more of an "all-or-nothing" conversation than a "limiting the amount of time spent" conversation, and for some people that step is too huge (or too huge at this point in time) and so you have to sometimes accept at least some of the crappiness of these people because trying to redirect or intervene in it entirely is a Sisyphean task.
