It's not that hard to figure out that this is an obsession that is Really Unhealthy for you. Let's just forget for a sec that you are related and point out that you are Not In His Life anymore. He does not want you in his life for oh...LOTS of REASONS!
He is Married and clearly does not want it exposed that he had a physical relationship with a minor. And a relative. It is over in his mind and the previous posters are correct; YOU NEED TO FIND THERAPY RIGHT NOW. You are only hurting yourself continuing to obsess over what he considers a grave mistake. You are not the mistake; his behavior is.
You pointed out that it was sexual only for him, which in real life terms means he took advantage of you, he used you to gratify himself with no regard for the damage he was causing you. He was the adult and that puts the onus of responsibility on him for making you feel as you do now.

My heart goes out to you as you face this abuse. Please seek help, it is possible to recover from such an outrage as this is.
Keep posting; many here will listen and try to help...