def get_family_ids_by_relationship(self, relationship_id=None):
pass<EOL>
Gets the ``Family``  ``Ids`` mapped to a ``Relationship``.

        arg:    relationship_id (osid.id.Id): ``Id`` of a
                ``Relationship``
        return: (osid.id.IdList) - list of family ``Ids``
        raise:  NotFound - ``relationship_id`` is not found
        raise:  NullArgument - ``relationship_id`` is ``null``
        raise:  OperationFailed - unable to complete request
        raise:  PermissionDenied - authorization failure
        *compliance: mandatory -- This method must be implemented.*