Is it permissible for me to marry my maternal cousin whom my maternal grandmother breastfed along with my maternal aunt?

It is not permissible for you to marry your maternal cousin whom your grandmother breastfed, because he has become your maternal uncle through suckling. For "what is prohibited by lineage is prohibited by suckling." And a sister's daughter is prohibited to her maternal uncle by lineage, as Allah the Almighty says: "Prohibited to you are your mothers, your daughters, your sisters, your paternal aunts, your maternal aunts, your brother's daughters, and your sister's daughters."