Speckled hen, I had the same thing happen with my last batch that hatched. 5 chicks, 3 boys, one obviously more dominant.
He also happened to be the largest of the young cocks.
When I was doing the rounds one evening I heard a terriable panicked sqwark comming from the broody cage.
When I turned round to see what was going on I was shocked at what I saw.
The dominant 6 week old roo had mounted his mother!
When he eventually got off she raised herself up, standing tall above him and pecked his little head 3 times, each time making him go lower and lower to the ground.
As it turned out his punishment wasn't severe enough, as 10 minutes later the exact same process repeated. Including the indignified head pecking.
I think his only saving grace is that he is a legbar hatched out by a peking/cochin bantam, so they are not related by blood.
So only a bit disturbing. He must be picking things up from all the big roos on the field