this reminded me of what someone said in another thread. due to the chronic sexual abuse, maybe burke was afraid he would be found out and he obsessed about killing his sister so she wouldn't tell. the idea probably ruminated in him for a long time. i think he knew his parent knew he was doing it because of the comment patsy made to her own father and the book that was purchased. I think they warned burke about this and he knew he would be in big trouble if he didn't stop. therefore, he killed jbr so she wouldn't tell on him. i have known of other children doing this sort of thing and the worry that lingers around it for the kids involved. just the fact that he went back to playing with his game boy or whatever he got for christmas. at 9 years old, he would have been crying their eyes out. he displayed unusual behavior. when i was reading doi, it was odd how they jr said that burke took his gameboy straight out of bed to the other house with him. no emotion, no tears. and then, he told his parents it was an accident. he had his parents eating out of the palm of his hand because of how it looked and they covered it up.

that's my theory.