code: def remove_sister(self, sister=None):
          
          sisters = self.get_sisters()
          if len(sisters) > 0:
              if sister is None:
                  sister = sisters.pop(0)
              return self.up.remove_child(sister)
docs: Removes a sister node. It has the same effect as
          **`TreeNode.up.remove_child(sister)`**
  
          If a sister node is not supplied, the first sister will be deleted
          and returned.
  
          :argument sister: A node instance
  
          :return: The node removed