Auntie mother-in-law

Tom:  “Our Aunt is coming on Christmas because she is family.”
Eva:  “She's NOT family?!”
Dad:  “Yes she is, she's part of our extended family.”
Tom:  “Yeah, she's your mother-in-law. Isn't she dad?”
Dad:  “Sister. Sister-in-law Tom.”

Better not tell her about this one.