code: def add_sister(self, sister=None, name=None, dist=None):
          
          if self.up == None:
              raise TreeError("A parent node is required to add a sister")
          else:
              return self.up.add_child(child=sister, name=name, dist=dist)
docs: Adds a sister to this node. If sister node is not supplied
          as an argument, a new TreeNode instance will be created and
          returned.