"Sexual abuse of natural children by their father is rather unusual. Generally it's a stepfather."
Actually, it's a lot more common that you'd think. It's very under-reported, and most cases never come to the attention of the police.
Child welfare workers, though, know about these cases, as do shrinks, etc.
It's a sad business, but it happens...
Actually I was in protective services for seven years. It could be underreported, but then we wouldn't know.
Reported cases most often involve a boyfriend or stepparent.