When I met my fiance he was married

Wendy
When I met my fiance he was married and I know some of you will bitch about how slutty that is but just hear me out. Him and his wife had been seperated for 3 years before I came along. Living in seperate houses and dating other people, only speaking when they needed to trade their son back and forth. When he falls in love with me he finally decides to get off his ass and finalize the divorce. Now his ex wife and I have a perfect relationship, we were just talking the yesterday about who will buy what off of their sons school supply list. Everything is perfect in our little happy family. We have a daughter now and I am pregnant with our second. But my MIL cant stand any of this situation. We are living with his parents for a short time inbetween homes and she just bitched today about how everything is about our daughter and its "sister this, sister that". She literally mocked us.... and thinks that her grandson deserves more attention. She undermines us right in front of her grandson and acts like he does nothing wrong when he is disobeying in front of the whole family. She eggs him on really is what it feels like. I feel like its because she blames me over the divorce or shes unhappy that we have children outside of marriage. I just dont know. Maybe someone else will have a better percpective from the outside. What can we do to get her to lighten up and respect us at parents?