<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def update_sibling_distance(self, sibling_distance, dest, topic):
 """Update the sibling distance for topic and destination broker."""

        for source in six.iterkeys(sibling_distance[dest]):
            sibling_distance[dest][source][topic] = \
                dest.count_partitions(topic) - \
                source.count_partitions(topic)
        return sibling_distance