Given a treeposition, return the treepositions of its siblings.

def get_siblings_treepos(self, treepos):
        """Given a treeposition, return the treepositions of its siblings."""
        parent_pos = self.get_parent_treepos(treepos)
        siblings_treepos = []

        if parent_pos is not None:
            for child_treepos in self.get_children_treepos(parent_pos):
                if child_treepos != treepos:
                    siblings_treepos.append(child_treepos)
        return siblings_treepos