Proceeding sibling

I think i can do it, but only with a couple of helper columns. I've attached a workbook that does what you wanted..

The helper cells do the following.

Row Number: an array formula that returns the largest row number that the current node apears in above the current row

If statement: uses the row number to check that the smallest number between whatever row we found above and the row 1 above the row we are on now isn't smaller than the current node

I cant combine them in to one huge, probably impossible to edit formula as the if statement brakes if its entered as an array.

Hopefully that helps.