Is it permissible to marry the children of my grandmother's brother, given that my grandmother breastfed her brother during her marriage to his father, and then she married his brother who fathered my father, and my grandfather's second wife breastfed me?

Your grandmother's foster sister is considered her daughter, and she is your foster paternal aunt, because she is your father's sister. Her children are your foster paternal cousins. Therefore, you are lawful for them to marry. Your suckling from your grandfather's second wife makes you your father's foster sister. This does not affect the lawfulness of marrying your foster paternal cousins.