<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_cousins_treepos(self, treepos):
 """Given a treeposition, return the treeposition of its siblings."""

        cousins_pos = []

        mother_pos = self.get_parent_treepos(treepos)
        if mother_pos is not None:
            aunts_pos = self.get_siblings_treepos(mother_pos)
            for aunt_pos in aunts_pos:
                cousins_pos.extend( self.get_children_treepos(aunt_pos) )
        return cousins_pos