rockaction
Footballguy
I have a SIL who isn't mentally healthy; my layman's diagnosis is borderline personality. She comes off as really nice but extremely fake and is easily the most annoying human I've ever encountered, and that even includes television (Richard Simmons, Fran Drescher, etc.).
She's especially manipulative and knows just how to find a person's weak spot and exploit it, much like a psychopath. My wife is kind and generous to a fault, so SIL uses guilt with her. Thankfully my wife has finally cut off contact with her and that has helped, though SIL still uses social media and other siblings to stir up ####. I'm getting close to calling her up and letting her know how I feel, which won't be pretty. One more event where my wife is left crying may do it.

Loving that. Best of luck, dude.