Is it permissible for the grandson of the elder sister to marry the daughter of the younger sister, if the elder sister breastfed the younger sister with her daughter, and breastfed the grandson himself, but not with any of the younger sister's children?

It is not permissible for the grandson to marry the girl because he has become her maternal uncle through suckling. This is due to his suckling from his grandmother, who suckled her younger sister (the girl's mother). What is forbidden by lineage is also forbidden by suckling. So, by his suckling from his grandmother, all of her daughters and their children become forbidden to him. He becomes a brother to her sons and daughters through suckling, and an uncle or maternal uncle to their children. As for the younger sister suckling from her elder sister, it has no effect on the prohibition.