How long do you have?
MIL, before our wedding, was lovely until SIL became jealous & set MIL off with lies about me. I was very young & endured hours of verbal abuse & DH didn't defend me. Just sat there. Looking back, I should have run.
Years of nastiness, ignoring my wishes with DC & feeding them loads of sugar & junk & being alternately lovely and nasty.
The worst, though was after my 2nd was born she came to my house to tell me 'everything wrong with my behaviour. She honestly thought I was going to sit calmly, listen & apologise. I've still no idea what her issue was because I asked her to leave. She had a full on tanty, screaming and yelling insults. THEN rang DH & told him to come home as his wife would be in a ball, rocking & crying. I was actually trying to settle both DC who'd she'd woken with her screeching (but my fault, apparently!). I just told DH I'm fine, I was expecting it one day, but I'm done.
He went to IL's that night & FINALLY told his parents to pull their heads in or her go NC. I did go NC.
What's worse is no one would pull her up. DH, FIL, SIL. I was still expected to be pleasant & pulled up on any infraction (I forgot to say please once).
I'm going to own it. When MIL passed I cried - tears of joy. FIL quickly remarried her friend, who is an absolute lovely lady & FIL is no longer nasty & rude. SIL has lost her back up so we barely see her. She'd become so bad her funeral was quite sparse. Her behaviour had pushed so many people away.