You should tell Steam you are thinking of taking legal action, as they are ignoring the sexual equality laws! Or, as I have the full original game (thank-you!), I can suggest you making her an adoption, so no blood relatives. The story could be exactly the same, and I am sure someone will come out with an "incest mod", but that's not under your control, so that's not your problem. Just change the beginning and some text, not sure it will be a lot of work, but then Steam might accept it, and then I expect you to get good sales, because of the story. And once the LGBTQ groups find out about it, will be get tons of free marketing/sales, I think! I think this game deserves to do well, hence my suggestions...! Good luck, whatever you do...
John