I know this isn't a direct answer to your question, but I've found it to be a much more efficient way to select siblings by simply selecting all the children of the parent, and specifying a unique class for the open as you have it there. So the code would be:
div h2 { } // apply the style for all "siblings" but really children
div h2.open { } // apply the style or cancel styles from the siblings
This works great for me and it doesn't require new css rules or anything special.
