This was when I was 18, the other two brothers were 10 and 12 at the time. It was found out by my mother (divorced from my father) who walked in on two of my other brothers engaged in fellatio. He would come over and they would get so sloshed together that this friend would end up spending the night.
This friend had been one of our dad's drinking buddies.
When I was 18, I found out that my brother (I'll call him 'T') had been sexually abused for years by a family friend.