Steam does not permit any lewd content in a game that has minors in it, from how the op makes it sound. So keeping her would mean, scorched earth no lewdness. :(
I don't think you get what he's saying, he's wants the game to have separate versions of it, one with 12 year chiaki on itch or some other website and one with 18 year chiaki on steam because of steams terms and such
