Therapy.. you need a therapist who specializes with BPD parents. You need to learn to draw HARD and firm boundaries that have consequences. Think of it as training for dealing with your future toddler (congratulations). Boundaries only work if you enforce them and impose consequences.
You tell your mother that comments about your in-laws are now 100% off limits. Let her know is she starts the conversation is over and you will talk to her a couple of days later. Then DO NOT ANSWER THE PHONE… silence her.. temp block her. What ever you need to do. She will pitch the MOTHER of all fits (imagine a toddler in a store). You 100% HAVE to ignore this and NOT give in.
Then in a few days you call…and chat. When she brings it up, you repeat she is to no longer EVER talk about your in laws and the next time she does it, you won’t speak to her for a week. Rinse repeat. Extend the no talking time longer for each violation. As long as you don’t cave… she will eventually. Right now she has trained you to jump