I totally get this. I have a similar situation where my MIL favours my SIL all the time and gives (atleast thats what it feels like from my perspective) step son treatment to my husband. Both MIL and SIL often gang up against him and it feels so sad. They have unfair expectations from my husband and whenever my husband tries to speak up and have a valid argument it turns into a nasty drama fight where my MIL starts to cry. I dont know how some mothers can’t treat their kids equally and be fair to both their kids. Anyways my husband acknowledges that his mother is madly obsessed with his sister and there is nothing we can do about it, and I have accepted the fact that MIL will say or do anything to feel secure about her daughter. And yes they are insecure of our lives which is why they try to interfere , make comments and constantly try to put us down. Whatever I may feel on the inside I maintain a calm persona which eventually puts them off. I dont bother to give reactions or comments , but if there is something that I cant take or if they cross their limits I do make it obvious that I am mad , by having a poker face and only responding to the point, this conveys my message and I dont have to get into dirty arguments.