I agree with anniebrooke. You have extreme negative thoughts about your brother. That's not typical unless some type of abuse happened and you may need to work through that. You don't want your relationships to be affected and you want to be your best emotional self. It's worth a little professional help to get there.
Andjdog429 does give a well known suggestion for eliminating negative thoughts. This does work for some people and worth a try.
good luck!
i had thoughts that were very impulsive but even though i am a teen boy i learned one way from my therapist is putting a rubber band on my wrist and when i thought about them i had to snap it against my wrist
Honey, I think in your shoes I would go see a therapist about the obsessive thoughts. Obviously you're a little out of the norm in thinking of forbidden or negative sexual things and not being able to get them out of your head, and a good therapist who works with obsessive-compulsive can really, really help. Ask your regular doc for a reference to someone good who deals with obsession and compulsion, and go see her. You have nothing to lose but your worries.