A "Joker-Quinn situation" is a type, or at least style, of a "love situation" where two people fall in love.
The situation involves a crazy or demented person and a normal or regular person who meet and they both fall in love, like Joker and Harley Quinn, when Harley was a psychiatrist before she fell in love with Joker.
Sally dropped a umbrella and it spilt her cup of coke on Shaun. He had to clean his shoes and was late for his tennis match. Sally got him into a twelve dollar situation
The scientific term for when someone gets stuck in a one-way hole (I.E. a Washing Machine) with no way to get out without the assistance of another party.
While doing the smex is the most common response to a Steppenbrother's Situation, it is not the only one.
Person A (Jack): Dammit Claudia, how the fuck did you get stuck in the AC unit!?
Person B (Claudia): Does that look like it matters right now!? Get me out!
Jack: Nah, as a high ranking member of the Mickey Mouse Club, I have to analyze this Steppenbrother's Situation so that it doesn't happen again.