I don't often post on this thread, but feel this should be mentioned. (I'll preface this with the opinion that RDI)
I believe it likely that although JR was probably abusing JB in some ways, the erosion of the hymen could have come from self-play or consensual sexual exploration with other children. For instance, if JR refrained from penetration, but engaged in stroking, petting or other manual genital manipulation that gave his daughter sexual pleasure and awakened her sexuality for the first time, no matter how inappropriately, she might have tried to stimulate herself the same way or even further, i.e., insertion of finger or foreign objects, vigorously rubbing her genital area against something that would, over time, erode the hymen further. I imagine if JR routinely spread her legs wider and wider during molestation, this could also cause tiny tears in the membrane and eroded it.
Also, if Daddy was diddling her and she liked the sensations and attention, she could seek out Burke or others to play house or doctor, and engage him in much of the same play. But because he wouldn't be aware of the need to protect the hymen, he was rougher with his explorations, and the hymen receded further on the vaginal opening.
IOW, I believe it's likely that JR was the first, but not the only one, who explored JB sexually.