My time to shine.
My MIL played the friend-then-enemy card for the entire six years my ex-husband and I were together. She'd pretend to be my friend (or at least friendly towards me) and then turn around to shit talk me to my husband. He's a childish momma's boy who's never going to grow up, so when mommy dearest says "jump" he does. When she started getting in his head about me, he never once thought twice about it.
She went through his bank account while we were out of town (after stopping in to stay in my apartment for over a week without notice - twice in two months), noted every penny he'd spent, where and when it was spent, and what she assumed it was all for based off of the store it came from. She then took this information to MY dad while my mom was at work (to cut her out of the conversation.. apparently she thought my dad was more reasonable?) and detailed this information to him. When we got home the next day, she started threatening to have me and my then-husband arrested and then started screaming at me to get a job to support her son. My spineless husband allowed her to stay in "our" home until the end of her trip a few days later.
She controls him while he's too absorbed in reddit, video games, and his need for validation and attention to see it. When she had enough of me, she planted the idea in his head that my family (and myself) were mistreating him severely all because I had the audacity to suggest he get treatment for his ADD and that he attempt to support himself through a green card and then a job.
She also decided that I have nothing better to do than to send anonymous messages to her and her estranged husband about her personal life.
Spoilers: I've got a million things better to do than ruin their lives as they do that just fine on their own. This divorce can't be finalized fast enough.
On a positive note, I've not been happier since he chose mommy dearest over being an adult.