Stepbrother Bob notices that Jim is jerking off under the sheets.
 Tells his father (Tony). Tony is a minister and disapproves.
 He thinks a punishment circ will fix Jim good. Local doctor is also
 elder in Tony's church. Agrees to circumcise Jim. A bit surprised
 that Tony wants Bob to watch "so he'll see it could happen to him,
 too, if he doesn't behave."

That night, Bob is to wait in the darkened bedroom till he
 notices Jim is beating his meat, then call his father and the doctor
 who will be waiting silently outside the door.

As much detail about Jim's cruelly tight cut as you like. The doctor
 notices that as they watch, Bob and Tony both have rock-hard
 erections. Maybe they've got into the whole idea?