There's a window of time in early childhood (up to about the age of 6) where, if raised with other kids, such individuals will develop a repulsion at the idea of having sex with each other. It happens with siblings, but it can also happen with people we're unrelated to - just growing up in the same household. It's called the Westermark Effect.

If anon wants a chance at this, his best bet would be to find twins that were raised apart from each other. Quite a tall order.

Worse, twincest creates much more congenital abnormalities. In this case they're both the same sex so it doesn't matter really.