When two women have had sex with the same man/men, they become clam cousins. This is a bond that can never be broken...unless they get jealous.
Variation: instant clam cousins - the resulting relationship due to a menage a trois.
"You see those two chicks over there? They're instant clam cousins. You know what that means? Oh yeah dog! Threeway!"
"My best friend Jullie and I just found out we're clam cousins!"