To add to your longer answer, it exists a lot in people who are related but don't meet until they're adults because of this thing called genetic sexual attraction. The theory is that basically all the instinctual feelings of affection you have for people who are genetically similar to you (your close family) suddenly gets switched on when you're already a sexual being, and so becomes sexual in nature. Doesn't happen for every separated family, of course, but it happens enough for it to be a thing.