Sayyiduna Mu'awiyah bin Abi Sufyan (may Allah be pleased with him) wrote to Sayyiduna Zayd bin Thabit (may Allah be pleased with him) asking him about the grandfather. Sayyiduna Zayd bin Thabit (may Allah be pleased with him) replied: You have asked me about the grandfather, and Allah knows best. This is a matter on which only the caliphs used to give judgement. I was present with the two caliphs before you; they both gave the grandfather a half with one brother and a third with two brothers, and if the brothers were more, they did not reduce him below a third.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12434