I'm of the belief that whoever molested Jonbenet probably killed her. With that in mind, I can only entertain the BDI theories if Burke himself had been sexually abused. The nature of Jonbenet's injuries (torn hymen, vaginal opening twice the normal size, etc) are more than a normally developing 9-year-old could ever inflict. The key word being "normal". Kids experiment sexually, but it usually ends at looking and light touching...not penetration powerful enough to change the vaginal structure.
If Burke was raping his sister, where did he learn such things from? There had to be some serious denial/neglect going on in that family. At the very least, the parents failed to protect their daughter from sexual abuse. If the tales of Jonbenet's urinary and fecal incontinence are true, she exhibited nearly every sign of child sexual abuse. Somebody knew something. For that alone the parents should've been charged.
(FWIW, I don't think Burke did it. I think the simplest explanation is the correct one in this case).
Remember there was a older, college -age half brother. He attended the local U of C and often spent weekends at the house. Interesting to note that MANY of JB's visits to the school nurse were on a Monday. Even more interesting- the school refused to provide JB's records of those visits. Coincidence? There are no coincidences.
This was the murder of one of their little first-grade girls. Wouldn't you think the school would want to HELP? Had to be something in those records that they did not want public. Because if it wasn't something incriminating, why hide it? Who cares about a tummy ache or skinned knee. I believe what they are hiding is directly related to abuse by someone (and yes, it could be BR) in that family.
An almost-10 year old boy KNOWS about sex. Most do not engage in sexual a activity, but a simple Google search on how young some kids are when they begin to have sexual contact will surprise you. Most boys that age can figure out what goes where.
Forensic experts did note her unusually enlarged vaginal canal. The coroner believed her injuries were from digital penetration. Many boys that age have adult-size feet and hands, or nearly so, even if they are not that tall. My 8-year old grandson wears a men's size 7 shoe.
If you read the GJ indictments, you'll see that it mentions "abuse leading to death". Someone in that house (and we have to consider EVERYONE in that house.
Kids sometimes penetrate, usually with an object or finger, rarely with a penis. This is a 6-year old girl. It wouldn't take much to widen her vaginal canal.