@BeanJen
Just you thread title is enough for me to say yep, got it:)
My SIL (brother's wife) is a nightmare. I and all the family were incredibly welcoming to her, she is a lot older than him and wants things her way, she's very entitled. My brother and I are incredibly close, they were having trying for a child but having trouble conceiving, she started pressuring him to propose by not good means, then she started getting crappy with me. Rude comments from her friends (excuse me, no), threats, all sorts a bizarreness. My brother was losing his mind because he didn't accept at all what her adult mean girls were doing to his sister, but then it's his wife who he has to live with. So he lost his mind and freaked.
It all baffled me at the time as it was so bizarre. Later I realised that she knew we were so close, she knew about all the problems conceiving, she knew he talked to me and she was (insulting to both my brother and myself) trying to avoid him somehow deciding to leave her because they weren't getting pregnant. As she knew he talked to me she figured I was a threat to her influence over him and might even tell him it's ok to leave. (It's f-ing not and knowing my circumstances which she did, no one sane would think I would ever tolerate or consider that).
He would never do that and had made it clear to her, actions and words, I would never tell a man to do that and quite honestly if my brother left a woman for that reason, I wouldn't talk to him for a very long time either.
She basically wanted our connection weakened so she could keep him for herself and be in control of him. I left it to him and didn't respond, my brother got incredibly upset though and it was difficult.
If your SIL is trying to interfere with your relationship, I would believe it. Some people are weirdly possessive like that.