A bit of advice would be greatly appreciated

My fiance and I have been together for almost 4 years. We have a son together. We are not perfect but overall, we have a happy relationship. His family is extremely toxic. I found out his mother has been telling him to hide money from me. She's telling him that my brother shouldn't be around my son because he called a daycare to complain about mask mandate - which resulted in him being expelled. Apparently that means he's a short fuse. Shes so toxic and always turns it around on me. I love my fiance but when is that not enough?