If my brother seems to live to give girls oral sex, am I wrong to let him do me?

When I was 15 my brother was 13 and his girlfriend dumped him because he couldn't get her off orally. Him and I were best friends and I was kiding him and said I'd let him practicse on me. We just laughed it off but then after few days he ask if I could teach him how to do it. I said no and it's kinda sick for him to even ask but he begged for mouth till I gave in. He did what I told him to which made him better at doing me than I could do myself. But that was 6 years ago we are both maried but he still want's to do me. I feel sick for letting him.