Mother in law

This is gonna be long. But I need some advice. So my mother in law and I have never fully gotten along. We did in the beginning for the first year we were together, but things fell off slowly when I learned some things about her boyfriend. I think she's a lazy mother by far. She puts her boyfriend(s) in front of her kids,and always has. My fiancé is the oldest (21) from her 4. And at 15 she let her current boyfriend lay his hands himand she lost custody of him. Well his second oldest sibling just turned 18 (he's completely deaf) and almost all of last year we had custody of him for the same reason because of the same guy. The next is 13 almost 14 and he lives mainly with his dad for yet again, the SAME reason. Then there's the little girl she's 5 he's her father. BUT he molests her and their mother STICKS UP FOR THIS MAN! Even in court under oath has LIED to keep him out of trouble. And he was convicted of the same thing in another state about 10 years ago (that's why he came to Michigan). But either way I don't agree with her parenting skills by any means but I've tried multiple times to make mends with her and make things okay for my finances sake. That's his mother. And my mom committed suicide 10 years ago, so it's importan tot me that he keeps a relationship with his mother. But I'm not sure what to do anymore. I'm stuck on keep trying with her or say to hell with it and leave it be. My fiancé hasn't spoken to her in close to year, has nothing to do with me, he just doesn't want anything to do with her. But I feel like if I made things right with her it would change the way he's looking at things. What should I do?