code: def siblingsId(self):
          
          if self._next_id is False or self._prev_id is False:
              self._prev_id, self._next_id = self.getPrevNextUrn(reference=self.urn.reference)
          return self._prev_id, self._next_id
docs: Shortcut for getting the previous and next passage identifier
  
          :rtype: CtsReference
          :returns: Following passage reference