Let him lick your pussy and forget about it.
you can stop the obsession by asking the wife. or, you could just flat out tell your husband about his behavior, and maybe they'll sort it all out... through words, fists, whatevers.
I know you're burning with curiosity and maybe a little lust, but the best thing to do here is nothing.
He and your husband and the other relatives will probably never be the same again if you tell on him.
You could, if you're really clever and have guts, make him think you're going to let him fuck you; then get him in some compromising position - without his clothes or waiting in a bed for you - and let his relatives see him that way.
It would certainly make a good confession here!
I had this problem too.
The brother in law was 18, I was 27. Problem is, I gave in. I felt so guilty, and still do. Problem is, I have to see him all the time.
Must admit, it was a mindblowing, incredible orgasm though. The kid definitely was the best I ever had.