Tell them to f#@k off.
Seriously. If the whole shebang falls apart because you aren't holding it together, too bad.
Family is overrated. There seems to be this mad idea that if you are related to someone you have to endure what ever bullshit they deal out. Really?
Are these the sort of people you would volunteer to spend time with if there was no blood tie? If the answer is no, abandon ship. Leave them to it. Bad behaviour flourishes when someone else holds the fort and encourages it.
That's probably enough mixed metaphors for this time of the morning, but you did ask for advice. Mxxx