I’m going to give a different input from everybody else. I can only speak personally as I’ve had incestual thoughts too during masturbation from a few years back. Like you, they were intrusive and ‘crazy making’. Over time I found out these incestual thoughts related to childhood sexual abuse by my sisters, parents. Once I realised I was sexually abused, the thoughts stopped. These thoughts are not fantasies that you want. I believe they come from your subconscious brain giving you a brief insight into what happened to you in childhood.

I’m not sure if the same relates to you as I can only give my story as happened to me. But I would probably suggest you speak to a therapist to talk about it further.