Bmt woes ... fiance mother switched up

Well me and my S/O have been together for a year and live together. He is truly my heart.I have three children from a previous relationship. I am doing well for myself and have been before he came into my life. But its better in all ways with him in it. Well his mother has been acting funny to straight disrespectful in a matter of his 3 WOT(befriend s before he left)....we met and began talking she started my telling me about my children , my middle baby in particular. and how she never wanted a relation ship with me.(mine you my children call her grandma)And so much more. Then she didn't approve of a facebook post and began replying bs on my page and also texting his boy saying im a swinger with my bestfriend....... I cant vent to him and stress him out while he is there so this is the only way I will not explode.....opinion s please.

Ps: she doesn't think im going to his coin ceremony but I am he told me I better be. Afraid of her starting drama. I can be civil as long as she is....