That prison idea actually sounds pretty cool, i honestly think you could do a lot with that depending on what theme/setting the prison is in, like standard, fantasy, or sci-fi.Make the ship a plane and the sister a daughter and you got Lewd Island.
Probably I would make a game where the MC was accidentally transfered to a woman's prison because of a mess up evolving his name and appearance. Two possible paths, one where you dominate the LI and rule the prison and another where they dominate you and have to become their little play thing
This site provides links to other sites/services, and does not store any files