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!