I wonder about that also, but I can't think of any possible explanation why PR (or JR, or even BR) would bookmark that word -- it just makes zero sense to me. If PR was staging this crime, that would be the last thing she would out in public (that incest was taking place in her family, in her home). The whole point in staging the crime, was to make it appear as if an outsider did it; but this bookmarked page goes completely against that theory, so why would it be done on purpose?
I definitely feel like PR bookmarked that page, but that perhaps she did it long before that night. If you are BDI, this is a very strong point of emphasis to prove that theory. I'm PDI, but even I must admit that. If someone was actually looking up this word, sometime before the actual murder occurred, that's very telling. And to me, only a person like PR (who majored in English, correct? So she's well read and surely did this before) would use a corner to bookmark a page.
One other scenario is, if it was done as part of the staging that night, is that PR was trying to set up JR out of anger about something. Here's where, if you're PDI, it proves that theory. If PR discovered JR acting inappropriately with JBR, and snapped that night, she would want JR to pay for his sick behavior -- so she highlighted that particular word in his dictionary in his office to make it look like he was preoccupied with that subject and to imply the inappropriate abuse that was going on, without actually saying it; and to point police towards JR, even though she was directly responsible for the murder.