Until the last couple years I had never even thought of BR as the suspect but this theory seems to pull together all the weird pieces of the puzzle. I've never been a PR did it theorist; I believe her life revolved around Jon Benet and she loved her dearly. But we have what seems like a frequently absent father who seems emotionally distant. A mom who loves her kids but is probably too sick a lot of the time to supervise them, and when she's involved with them, the focus is more on JBR than BR. Some kind of dysfunction and sibling rivalry maybe grew out of that, along with possibly other issues BR had, and maybe even something happened Christmas day that really bothered him. JBR got better gifts, got more attention, etc.
If he did the head bashing and the garrotte (possibly the paintbrush too) that leaves the parents with just the ransom note and the cover up. I could never imagine how a parent could strangle their child to cover up for the other one bashing them in the head. Why not just claim an accident? (Kids were playing, JBR fell and hit her head, or BR accidentally hit her with something.) But if your child garrotted his sister and perpetrated a sexual crime on her, then I can see the parents going to such lengths to protect him. I don't sympathize with the parents. If they knew there were serious problems, they should have been addressed long before it came to this, but it relieves us of having to imagine PR or JR doing the garrotte and the paintbrush. It leaves them with a horrible and unimaginable tragedy with one child dead and one who caused the death.
Also, I wonder about the wine cellar door that had the lock high up at the top and a chair nearby. Could BR have used the chair to stand in to reach the lock?