Struggling with groinal responses and it’s killing me.

I (18) have intrusive sexual thoughts about my sister (16) and I’ve started to get groinal responses to them and it’s driving me insane. I will just have thoughts about a random female body, without even really a head or any distinguishing features, and then my brain will just say ‘This body is your sister or this is a minor, if you feel any thing at all when you imagine interacting with this body, you are attracted to her.’ I feel paralysed whenever those thoughts come into my head, because whatever I do I’m screwed. If I push the thoughts away, they just keep coming back. If I interact and feel nothing then I’ll just get anxious again and keep checking until something does happen because I feel like I need to be absolutely certain before I move on. And then if I do feel something then that sends me into an anxiety spiral that takes over my mind. Last night I manipulated myself into imagining taking off my clothes in front of this person less body and I felt something down there, and I couldn’t sleep properly the rest of the night. The feeling almost felt like a warning from my body, like, ‘Stop, stop it right now I don’t like it.’ I drive myself nuts because no matter what I say, my brain will tell me that I’m just making excuses and ‘when will I just admit that I’m attracted to her.’ It makes me feel so awful, and when I’m actually around my sister I feel fine and don’t have any anxieties because they seem so ridiculous, like, of course I wouldn’t want to do that. But dear God this is tearing me apart.