Ok, I got it. Here's the code I came up with:

**=======================================================================
** Removes all the siblings of the passed node.

LPARAMETERS toNode

LOCAL oParentNode, iNodeIndex, oLastNode

** Grab the parent node, and the last sibling
oParentNode = toNode.PARENT
oLastNode = toNode.LastSibling
iNodeIndex = oLastNode.INDEX

IF oParentNode.children > 1

	iNodeIndex = toNode.FirstSibling.INDEX

	** Loop through all the siblings and remove them.
	** then indexes may change so we always need to get a new
	** reference to the LastSibling. This loop leaves the
	** last sibling.
	DO WHILE oParentNode.children > 1

		THIS.Nodes.REMOVE(iNodeIndex)
		iNodeIndex = oLastNode.FirstSibling.INDEX
		oLastNode = oLastNode.LastSibling

	ENDDO

ENDIF

** Only the last is left, so whack it.
THIS.Nodes.REMOVE(iNodeIndex)

>Try THISFORM.oTree.Nodes.REMOVE(iNodeIndex)