When Sayyiduna Ibn Abbas (may Allah be pleased with them both) was in Basra, he wrote to Sayyiduna Ali (may Allah be pleased with him) that I have been asked about the issue of a grandfather and six brothers. Sayyiduna Ali (may Allah be pleased with him) wrote in reply that give the grandfather one-sixth (sudus), and after that do not give anything to anyone.
(That is, one-sixth is for the grandfather, and the remaining five-sixths are for the brothers.)