Ibn Abi al-Zinad (may Allah have mercy on him) narrates from his father (may Allah have mercy on him) that he said: We have heard that if the one from the mother's side is the nearer of the two (to the deceased), she is more entitled and the sixth is hers, to the exclusion of the one from the father's side; and if the two of them are of the same degree in relation to the deceased, or the one from the father's side is the nearer, the sixth will be divided between them in two halves.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12364