OK, I've had another think and drawn out a little diagram.

I think I understand.
Richard Corbett's father was X Corbett.
Richard Corbett's mother was X Halfpenny. Her brother was Benjamin Halfpenny, (thus making him Richard Corbett's uncle).
So we need to find (or do you already have any of these?)
Richard Corbett's birth?
Richard Corbett's parent's marriage?
Richard Corbett's mother's parents and siblings (one of which is Benjamin Halfpenny)
I think that's right ... and hope I've explained it clearly enough.
It might also help to locate Mary and the children on the 1841 census, if she was still in England. Do you know the date she and the children came to Australia?
All this might be incorrect of course if Benjamin Halfpenny was a great uncle, or a husband of his father's sister (ie uncle by marriage) ...

Gee I hope not.
