Had it with my MIL

My SO and I live together and he barely calls texts visits his mom. Her and I are not in the best place (haven't talked for 1 year) but I still encourage him to call, text and visit her as much as I could regardless of the beef, he just doesn't and I can't force him. I'm 5w3d and she doesn't know. But she sends him texts throwing indirects towards me all the time and it's upsetting me. She says things like "you only get 1 mother" like trying to say you can have another gf but you only have one mom. I'm very sensitive and it makes me mad. Yeah you only got one mom but I'm gonna be taking care of your son when he's an old man & you pass away... I haven't spoken to her because everytime we makeup it's like a curse towards our relationship. What should I do? ....