Struggling to resist temptation to do despicable things

What follows may upset some and I do not wish to do that. If you do not think you can handle a post that will talk about incest during childhood/teenage years, please don't read further. This is serious and I have never shared my inner struggle until now.

My earliest memory of my fascination with my little sister is 2nd grade. She is 3 years younger than me. We would take baths together for quite a while as we were a big family with 2 bathrooms. I tried experimenting with her then, and at that point I would say it was still harmless. Many incidents followed after that, all the way until about 8th grade. I tried to get her in the bathroom alone with me, in the backyard, even in the car once, each time I wanted her to see my thing and touch it. Due to other sexual stuff with a neighbor girl, I began going to therapy around 4th grade. I also had stuff happen with girls at school, and even my guy friends. It seemed normal to me.

My best friend was also into doing stuff with my little sister and he twisted my mind, and got me to do more evil. Eventually it all came to a halt when she told my parents, and honestly I think they didn't believe it. But I went to therapy for a year or so and from 8th grade to graduating high school I didn't do anything. I thought I was better.

However, during those 5 years of not acting on my urges I was satisfying them by watching incest porn. A lot of it. It's the only thing that works for me, and suppresses the thoughts I hate. I have been into mother-son stuff as well as sibling stuff, and it doesn't help that Game of Thrones showcases incest and it's only the most popular genre of porn on the internet. So finally, to my great shame, I fell to my urges last summer before college and began again. I let my sister "catch" me masturbating, and the cascade went from there.

I could type pages about my struggle but I will end here for discussion and helpful advice. I want to fully cure myself. My sister and I talk from time to time but we do not have a healthy sibling relationship, for obvious reasons. I think she represses a lot of what I did to her when she was younger. I feel horrible every day and wish I could go back and grow up normal.

I've been thinking about your thread for a few days because it is clearly an on-going issue that your sister has fallen victim to and is seemingly in danger of falling victim to again. Truthfully I don't really know what to advise. How has this issue been discussed between the two of you? From your description it sounds like your behaviour - which may have started off as experimentation - has been coercive and abusive. You need to be dedicated to stopping this and I think you need to get yourself back into therapy. Please at the very least take a look at this link:
https://www.stopitnow.org.uk/