Sorry to disagree but, in the US if a woman and man marry, and that marriage has children from a previous marriage (from either husband or wife) the step parent who engages in sexual activities with those step children are committing a crime. The courts would call it both Incest and child molestation, same as any European Country. Once the Non-Blood related parent engages in any sexual activity with the step child they can be prosecuted for a sex crime.
Understand, however, I am speaking of Real Life, not a game. In a game, any laws could relate, it is up to the developer. for this game, yes PayPal, Patreon, or any other pay site could ban games from their site that includes incest, underage sex, etc... That is up to the website.
My previous comment was to only explain that in Most Western World countries, incest can cover a wide range of familial sexual activities, from Father/Daughter, Mother/Son, siblings, cousins Aunts and Uncles, etc. And as far as I have ever been aware of, Incest in most of its forms is illegal (in Real Life).