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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 19, 2018
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Jan 27 Word of the Day
Referring to the heavy unfashionable winter jacket and patterned mittens Sanders wore to the inauguration of Joe Biden.
Its brutally cold outside this morning so be sure you're Bundled Like Bernie!
by Talk2me-JCH January 24, 2021
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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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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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