When a male and a female have both had sex with the same male.
Person 1: Did you hear Louie and Halee both hooked up with Johnny?
Person 2: That is crazy, they are for sure cock-kin
When a male and female have slept with the same female.
Person 1: Did you hear Halee and Louie both hooked up with Chrissy?
Person 2: Yea, they are defintley slippery siblings!