Return ancestor siblings based on Id

Hi guys,

Another chapter in this tragedy.

Question: 

The expression '//tree_node[@id=9]/ancestor::tree_node' returns a nodeset
containing node with id(7). I have verified this with xpath visualizer.

How do I return the ALL siblings of this node also.

The result being
Node(id=7)
Node(id=8) : sibling of Node(id=7)
Node(id=9) : sibling of Node(id=7)
Node(id=14) : sibling of Node(id=7)

HELP APPRECIATED!!!!

ALL TO0 FAMILIAR XML STRUCTURE:
<tree>
  <tree_node id="7" value="Test Level One A">
    <tree_node id="8" value="Test Level Two A"/>
    <tree_node id="9" value="Test Level Two B">
      <tree_node id="11" value="Test Level Three B"/>
      <tree_node id="10" value="Test Level Three A">
        <tree_node id="12" value="Test Level Four A"/>
        <tree_node id="13" value="Test Level Four B"/>
      </tree_node>
    </tree_node>
    <tree_node id="14" value="Test Level Two C"/>
  </tree_node>
</tree>