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1.
Two women who have slept with the same man in their past.
Judy and Gail were Eskimo sisters since they both went to bed with Ronnie during their freshman year.
by Joe Corey February 14, 2004
 
2.
Two females that have had sexual intercourse with the same person.
"Dude you fucked Jacob too?! We're eskimo sisters!" :D
by bricmeow August 11, 2011
 
3.
A woman who has shared a sexual partner with another person and the two are on friendly terms. In today's day and age, the eskimo sibling could be a male or another female...you just never know.
Jenna: "Hey do you see that girl with the tramp stamp? I totally did her last week at Michelle's party."

Brad: "What? I was inside her the night before. Give me a high five. You're my eskimo sister!
by EskimoSibling January 10, 2012
 
4.
Two or more women who have performed fellatio on the same man (not necessarily at the same time) and swallowed his cum.
"You sucked Robbie off too? Now we're Eskimo Sisters!"

Can also be used in the context of the man referring to the women as his Eskimo Sisters:

"Hey, those girls at the bar are cool--let's join them"
"How do you know them?"
"They're my Eskimo Sisters."
"Nice!"
by JacksonDollup October 04, 2013