When two women have had sex with the same man/men, they are Clam Cousins. It is the same concept as two men becoming Weiner Cousins or Eskimo Brothers after sleeping with the same woman/women. Except, unlike men, who become closer than brothers after discovering their bond, women who are Clam Cousins must become sworn enemies.
"Did you hear that Angelina found out that her boyfriend used to sleep with her friend Jennifer back in the day?"
"Uh-oh. Now they can't be friends anymore. They are Clam Cousins!"