Ames, thanks for your answers.
There's just one problem. That sexual wound (I hate the 'v' word too, btw), according to the medical evidence, had to be inflicted before JBR was dead. Because it bled, at least enough to leave blood on the size-12 underwear. It's possible that it bled more than that, and made it necessary for the killer to remove whatever underwear JBR had been wearing before, and to wipe her down the way he did.
If the wound had been inflicted after death, as staging, it wouldn't have bled.
So, the killer inflicted the wound. Only the killer could have done so.
Now, to me, there are three possible reasons for this wound.
1. Perverse gratification. No evidence supports this, though; there is no evidence from the crime scene that this really was a sex crime, despite the way it looks. (For example, there is no semen anywhere on or near the body, there is no sign that JBR struggled with her attacker, etc.)
2. Staging. Ruled out because of the above, the fact that the wound had to be inflicted before death.
3. Attempt to confuse evidence of prior molestation. In my mind, this is the most likely reason this injury was inflicted; the killer knew JBR was being abused and, further, knew that this evidence would in some way be dangerous to him/her; so he/she decides to inflict this injury so LE will not be able to state with certainty that JBR has been abused prior to the night of the murder. But this means, as I've said before, that either the killer is the molester, or the killer knows with absolute certainty that the abuse is occurring and decides to corrupt the evidence of the abuse.
What bothers me about that second possibility is the 'why'. If PR, for instance, is the killer, and the killing has nothing whatsoever to do with the molestation, why would she hide the evidence of that? Who is she covering for? What will this do for her?
I find it much easier to believe that this whole crime involved the molestation, that the molester became alarmed at the events leading up to Dec. 25 (in particular the 911 call on the 23rd and JBR's 'I don't feel pretty' moment) and that something occurred on the night of the murder that made the molester believe that he/she was in immanent danger of being exposed. (It could be as simple as JBR disobeying the molester, and saying something like 'I don't have to do what you say,' which the molester interpreted as a veiled threat.)
I tend to believe that if anyone of the three people at home was the molester/killer, it was JR--but that's probably a whole post of its own.