Incest aversion actually has more to do with being raised together. There's something called the Weintraub effect, I think? Something they found on Israeli kibbutzes. Basically, raising the kids in a massive group, as basically family, led to almost no marriages between kids from the same kibbutz. Because it felt incestuous.

There's also a similar thing with Chinese brides who were sent to their future husband's family at an early age and raised together. Growing up as effective siblings tended to ruin the subsequent marriages.