code: def update_sibling_distance(self, sibling_distance, dest, topic):
          
          for source in six.iterkeys(sibling_distance[dest]):
              sibling_distance[dest][source][topic] = \
                  dest.count_partitions(topic) - \
                  source.count_partitions(topic)
          return sibling_distance
docs: Update the sibling distance for topic and destination broker.