It is narrated from al-Sha'bi, and from Humayd from al-Hasan, that both of them said concerning the mother of the maternal grandfather that she does not inherit.
It is narrated from al-Sha'bi: Her son, who is nearer than her (to the deceased), does not inherit, so how can she inherit?
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12358