According to the Patreon rules, if siblings are involved in a sexual act (no matter how, even if they're just watching), it's incest. As for getting banned for this, it can vary. It probably requires someone to complain about the game first, and then it all depends on how well the developer fights back, but there are cases where games with incest are quietly available on Patreon, and there are cases where even a third-party incest mod has been banned.
So even if Enna makes up with her sister, the sex will still be strictly separate. The dev already had problems with Steam on the previous game (one character seemed too young to them), so they are unlikely to try their luck with Patreon.