(b)in the event of the pension not becoming payable under clause (a) the pension may be granted-(i)to the father;(ii)failing (t) above to the mother;(iii)failing (i) and (ii) above, to the eldest surviving brother below the age of 18;(iv)failing (i) to (iii) above, to the eldest surviving unmarried sister;(v)failing (i) to (iu) above, to the eldest surviving widowed sister; and(vi)failing (i) to (u) above, to the children of a predeceased son in the order it is payable to the children of the deceased officer under (a), (ii), (iu) and (iu) above.