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A sister who by blood is a sibling, but happens to play the role of Mother figure as well. (Sibling Co-Parenting)

This can occur for a variety of reasons such as cultural background, parents work schedules or simply having a large family.
Example 1: My sister-mom has helped me raise my grades, helped me focus on my future and is also my best friend.

Example 2: Teacher - "Hey, is that your mom over there? Is she picking you up from school today?"
Student: "Oh, no, that's my sister-mom."
Teacher: *confused look*
Student: "Well, she is my sister, but she also acts like my mom"
by WordWatch January 20, 2018
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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.
by jillyjillz April 13, 2011
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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"
by zeilgarsiulgfiSDoiuSEibugsEigu January 28, 2012
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