mercedes was engaged to a joker who was killed about ten or so years ago. she's sort of been affiliated with the jokers ever since, and when she became a doctor, she lent them a hand when they needed her to thank them for keeping an eye on her. she basically works for them under the table now when they can't bring someone to the hospital. i thought it would be interesting for the girls to be sisters but have a very difficult relationship because of their affiliations. and since both olivia and meghan are mixed race, we could say that they're half siblings and they share their caucasian father?