My MIL didn't like me because, as DH and I developed a relationship, DH developed an understanding of everything wrong about his childhood and his relationship with her.
I grew up with an abusive parent - emotional, psychological, physical, social, and sometimes financial. As DH and I learned about each other in that way you do when your relationship is deepening, he began recognising his own experiences in what he learned of mine. He learned it's not normal to be locked in a room or denied privacy or physically dragged out of bed at 2am and put out onto the doorstep or to have plates or shoes or hammers thrown at you, to be slammed against walls, be told you're worthless/ungrateful/unwanted, and so on (there's more to the abuse but I don't want to go into that level of detail).
There was no big confrontation but he began to slowly distance himself from her, quietly withdrawing until he eventually went NC. At that point he told her why and that he didn't want her in his life and certainly didn't want her around any DC we had.
She blamed me and, to this day if asked, will say I turned him against her. I filled his head with lies. I considered myself above them and too good for their family. That I'm spiteful and selfish. I'm afraid to share him with them because I know I have no chance of holding him if he knows he can escape to his family. I'm m cutting him off from them because I want him to have other options but me.