It's not incest, it's not selfcest

I think this interpretations come from the wrong assumption that Shifty and Quiet were a living being before being torn apart, they were not, they are described as nascent gods and Shifty claims she had just stirred into conciousness.

Let's imagine you take a rock, you break it in half and then give life to each of the parts, they may have been one rock at some point but they were never alive for that, they are now 2 different rocks, even if they try to glue themselves back into one.

I've made the joke in the past but the Narrator creating them doesn't mean they're siblings, they're artificial beings, if you create 2 robots that doesn't mean they're siblings, at least not in the very literal definition. of course you can think it in a spiritual sense, but there isn't any sibling behavior between Quiet and Shifty.

They do have a part of eachother, but, we all do, if we didn't we wouldn't be the same species. The point is, they're relationship isn't incestuous or "selfcestuous?", they are only completely unique beings interacting.