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1.
Term used when two girls (well, obviously) cut themselves and mix their blood. Can be drinken or just pressing the cuts against each other.
I acquired a blood sister on Feb. 18.
by Charlie Storm April 21, 2005
 
2.
The female equivalent of a blood brother.
Blood sister can refer to one of two things: two females related by birth, or two or more women not related by birth who have sworn loyalty and friendship to each other. This is usually done in a ceremony, known as a blood oath, where the blood from small cut is mixed and drunk.

Much more than just a Best Friend, Blood Sister is your hetero life mate in a non-homosexual way.
Blood Sisters share anything and everything; they have no secrets from each other. She is number one in your life, despite significant others and family. Hell, she IS your family. A Blood Sister is your other half.

Also known as Soul Sister.
Person you are so close with you have "inside" jokes with and code words. Person that you can just look at a certain way and knows EXACTLY what you're talking about.

Person no distance can separate you from because you're close in heart.

Tahun: Let's cut our hands and mix blood so we become blood sisters like in Winnetou!
Keliona: Yea let's do it!
by Tahun June 20, 2011
 
3.
When more then two girls have that special present from mother nature called the period on the same day and share the cramps and the mood swings together.
Charity and Hilary hung and found out they were Blood Sisters!
by Alex Haaaas March 09, 2009
 
4.
two lesbians having sexual intercourse while both on their menstrual cycle.
The two blood sisters better not stain my sheets!
by Agent Filth and Little Bill November 06, 2010