Is it permissible for a girl to marry her male cousin (her mother's sister's son) who was nursed by her mother's mother (her grandmother) five or more times, which would make him a brother to her youngest maternal uncle?

If your male cousin breastfed from your grandmother, he becomes your maternal uncle through suckling, and you become his sister's daughter through suckling. It is not permissible to marry the daughter of a sister, whether through suckling or lineage, because Allah Almighty says: ﴿Forbidden to you are your mothers, your daughters, your sisters, your paternal aunts, your maternal aunts, your brother's daughters, your sister's daughters, your foster mothers who suckled you, and your suckling sisters﴾, and because the Prophet, peace and blessings be upon him, said: "What is forbidden by lineage is also forbidden by suckling."