Actually there's no difference officially between Patreon's rules and those on Subscribestar regarding incest. It's just Subscribestar are not as zealous when it comes to enforcement of their rules as Patreon seems to be. My guess is, Subscribestar would only move on this, if someone made an official complaint about it, at which point they could if they so wished, remove content that contravenes their rules without any notice. This obviously doesn't happen very often, otherwise we'd have heard about it, but it's interesting to note, that the general belief that Subscribestar have different rules to Patreon on this issue, is incorrect.
In California, where Patreon is based, sex between step-relations is not classed as incest. But in many other US states and some European countries (including here in the UK) it is. Which is probably the main reason why Patreon have decided to class it as incest (because it affects a lot of their potential customers)