What causes two family members to become incestuous?

I asked this question on r/askpsychology and the automod removed it and told me to post here.

I’m working on a story where the main character’s parents are siblings and has health issues because of it, and I want to know what background I should give them. They were raised together, so it wouldn’t be because of the distance. Is it possible for two siblings to just randomly start liking each other, or would there have to be family trauma, the way they were raised, or other environmental causes? Furthermore, how would being raised by parents who are siblings affect you psychologically?