Ata reported that Ali (may Allah be pleased with him) used to treat the grandfather as a father.
Some of the people of Iraq rejected Ata's report of this from Ali. What is authentically established from Ali (may Allah be pleased with him) is that he used to make the grandfather and the brothers share together, and perhaps he treated the grandfather as a father in some other ruling. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12426