I think I have pocd I'm 16 I keep getting these weird scenarios of me having sex with my ill bro I'm straight&disgusted by this. I get minimal arousal?

Fantasy vs Reality. If these are infrequent fantasies then I wouldnt worry, especially if you have not acted on your urges. However, if they are more frequent and intense within a 6 month period you should seek professional help. Have you or family seen or have been victims of sexual abuse? There is help for you.