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Sister Code is an excuse one sister will use to not have sexual relations with someone who has already slept with their sister. The Sister Code doesn't exist and is only used when the girl is an actual prude. If a girl uses this term with you, she is just trying to politely say "You're a fat slob and you need to move on!".
I do like you but I can't have sexual relations with you because of "Sister Code"!
by wisdomson June 12, 2014
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