This is a weird situation, but I was married to my husband for six years before i met MIL. He does still talk to his father and his sister-in-law, and everyone, to some degree, thinks the mother-in-law is being dramatic. When they visited the father-in-law and OP went to lunch with him and her sister-in-law, lo and behold they ran into the mother-in -law. They had an awkward lunch, and when her mother-in-law learned the truth later, she was quite upset. I guess SIL told her when they got home, and MIL lost it and was screaming and throwing shit around.