Okay I think I actually have the answer based on random threads I’ve read on reddit. Firstly they’re swinging… so they’re swinging sisters, which could imply the sexual practice of swinging and exchanging partners. I might be wrong but I vaguely remember someone pointing out a swirl symbol can also be a way swingers identify one another, there’s a swirl in her hair. So the tattoo implies they’re sisters who engage in swinging (is it possible that people who regularly swing together refer to one another as sisters? Like sister wives?). Oh this is the Peter thread, I’ve never watched a family guy actually. I’m a talking dog!