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one of two (or more) women who happen to fall on the same menstrual cycle. Menstrual sisters often times find comfort in sulking, eating, complaining, and causing a scene together.
My menstrual sister, from the stall next to mine, threw a tampon at me as I kicked my door in anger.

As a good menstrual sister, I shared my chocolate as we bled and weeped together.
via giphy
by PrincessGaylordFocker December 11, 2016
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