Ibn Tawus narrates from his father that Ibn Abbas (may Allah be pleased with them both) said concerning the sixth from which the brothers block the mother: it is for the brothers, not for the father. The mother's share is reduced and it goes to the brothers.
Ibn Tawus said: It has reached me that the Prophet (peace and blessings be upon him) gave them the sixth. I met one of the children of the man whose brother was given the sixth, and he said: We learned that it was a bequest for them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12298