I maintain an old cms that stores the id of the parent, left sibling, and right sibling.

Making any changes to the order or grouping is wrought with peril because you never know if everything is going to be updated properly. When it isn't, it's an ordeal to correct.

Having a parent id and sort order is straightforward and about as simple as it gets. I really don't see why you'd need to do it any other way outside of a thought experiment.