When your dad has sex with a woman and said woman has a girl for a child. Then the dad fucks the daughter and you are born. Motha fuckin sista mom
guy #1 "Dude your mom is hot"
guy #2 "That's my sister"
guy #1 "Oh shit, you got a sister-mom"
When someone's mom looks so young she could be their sister.
Jenny is coming to the party and she is bringing her sister-mom. I saw Chris shopping with his sister-mom.