Here's a couple for y'all.

Names: Minerva, but likes to be called Mimi (l), Alexander/Alex (r)
Age: Mimi: 20; Alex: 16 (if you want you can make him 18 and just a bit short/scrawny)
Traits: Mimi: Acts gentle, sisterly and soft-spoken, but has a *very* kinky hidden side. Alex: Kind and somewhat naive, he's eager to please and always goes along with Mimi's ideas.
Premise: The two siblings are in an incestuous relationship since Mimi took Alex' virginity for his 15th birthday. However recently, they've been found out by their parents (or so Mimi says), and it's only a matter of time before their lives are ruined from the fallout, so the only option is to fuck one last time and then die (she claims).
Scenario: To avoid a messy suicide and needless pain for her brother, Mimi turns to you(any gender), asking you to snuff them both. She's not too picky about the specifics, as long as Alex gets to cum inside her one last time; beyond that you're fully invited to either watch or take part in their last time, however you want.