My apologies for kind of "re-posting" this, but it's my theory du jour that I initially dashed off on the fly. I've re-written to clarify, and I would really like to hear if it can be debunked:
At some later point after eating a piece or two of pineapple, JB and BR got into an altercation and BR hit JB on the head with the heavy flashlight. Unlike any previous times he had hit her, she went down and didn't move. At age 9, he may or may not have understood she was unconscious. Whatever he thought was going on, he most likely was scared of her waking up screaming and crying which would bring one or both his parents downstairs and he'd get in trouble for hitting her.
So he got hold of her by the back of her nightshirt and tried to drag her (because he couldn't lift her). This could have been (1) from the kitchen downstairs to the basement (if the altercation happened in the kitchen), or (2) from one place in the basement to another (ending up in the "wine cellar"). His reason would be to hide her until she woke up.
Trying to drag her by the nightshirt was not working too well (but it caused the triangular red mark on front of her neck). So he got the cording and put it around her neck in a slipknot (typical knot a kid would use for just about anything) and tried to drag her that way, but the cording kept slipping through his hands.
So he wrapped the other end of the cording around the piece of paintbrush handle to make a "grip" for his hands. [Or it's possible the cord with paintbrush grip was already lying around in the basement, having been made and used for something else.] He was then able to drag her body (including possibly down the stairs) to a place where his parents wouldn't immediately see her. The effect of this dragging tightened the cord around her neck and effectively strangled her although he was not aware of that.
He waited around a while for her to wake up. Possibly he engaged in some sexual curiosity -- she was asleep and wouldn't know .... He poked her a couple of times with the train track trying to wake her up, but that didn't work. So he got a blanket and put over her, either to try to kind of keep her warm or possibly to help hide her in case the parents looked.
And went upstairs to bed because he didn't know what else to do. Scared to go wake up his parents because whatever was wrong with her was his fault since he hit her. And he probably stayed in bed, scared stiff, when all heck broke loose, until whatever point he finally came downstairs. At some point shortly thereafter, he confessed to his parents about hitting her and then hiding her, and they came up with their plan for the ransom note and whatnot to try to protect him.
I thought his behavior during the Dr Phil interview/s was clearly nervous, but what struck me was that his smiling was "appeasing", like a dog that tore up the living room while you were gone and knows it's in trouble but doesn't really understand why. I don't think he meant to kill his sister -- I do think he hit her out of an in-the-moment rage reaction and had no idea what he'd done or that his following efforts to move and hide her ended up effectively sealing the issue and killing her (although the blow would have apparently resulted in her death anyway).
Her body had abrasions on the back consistent with having been dragged, and any of the adults could have/would likely have picked her up and carried her rather than drag her.
How many of the actual elements of JB's death does this satisfy?