@abc.abstractmethod<EOL><INDENT>def assign_relationship_to_family(self, relationship_id, family_id):<DEDENT>
pass<EOL>
Adds an existing ``Relationship`` to a ``Family``.

        :param relationship_id: the ``Id`` of the ``Relationship``
        :type relationship_id: ``osid.id.Id``
        :param family_id: the ``Id`` of the ``Family``
        :type family_id: ``osid.id.Id``
        :raise: ``AlreadyExists`` -- ``relationship_id`` is already assigned to ``family_id``
        :raise: ``NotFound`` -- ``relationship_id`` or ``family_id`` not found
        :raise: ``NullArgument`` -- ``relationship_id`` or ``family_id`` is ``null``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure

        *compliance: mandatory -- This method must be implemented.*