I think he means that he was messed up mentally/emotionally before it happened. I'm guessing he feels immense guilt for having engaged in sex acts with his sister.

I never had siblings or even friends growing up, so I can't say I've ever been in OP's shoes. However, I have had (former) friends confide to me that they had engaged in sex acts as young children with siblings, cousins, friends, etc. They would all tell me that they felt suicidal and immensely guilty about it, some of them would even tell me they felt like rapists. In turn, I tell them...that, well, it seems like it's something that happens because humans get curious about others' and their own bodies, even at a young age. I never experienced this myself, as I said, but I've heard and read about these accounts online. I think there's behavioral therapists that say a lot of this is common amongst children. I won't get into it further, but there's lots of material about this subject online, along with support groups that discuss it.

Thing is, people do get judgmental. And people love to shove your face into the mud when you're already down. Thus people get afraid of how they'll be judged if they admit to something like this. Like I said, many of these people I've spoken to would tell me they felt like they did something immoral, disgusting, illegal, and reprehensible on the same level as outright rape and torture (tbh, the way they'd describe it, it seemed mutual and the sort of "touchy feely" stuff doctors describe as experimental petting aka "playing doctor")...