She should never go to that house again. If her husband asks why, tell him that it's:

"Because you won't defend me when I'm assaulted by SIL. I'm not overreacting. She's fucking insane. I like your brother fine, but I will not be in the same building as his psychopathic wife. I'm not overreacting. You're underreacting and it hurts that you won't take my side on this."

