When she spots them, she sees that they''re being a little too touchy-feely with each other.
Mercedes is worried at first because they got caught, but when Alina tells them that she's also a lesbian, Christie invites her to join them and after all, her mother did tell her to make the guest feels welcome! Her mother is a lesbian! As a result of this, she'll have to live with both of them for at least a year.
As she watches her stepmom make out with Christie, she starts getting turned on.