As you noticed from the comments, abuse by a sibling is not as uncommon as parents wish it would be. You perception of "sibling rivalry" is widespread. That said, I am sure you were a wonderful parent in many other areas..."good enough" to use your description. We would all like to be "perfect" parents and even if we come close to that goal, our children will find something to fault us for. How your daughters are now may or may not be related to their interactions as children...raising children is complex. Try to stop hating yourself for this one aspect of it.