Is it permissible for the daughter of the questioner to marry the son of his maternal brother, who was not suckled by the questioner's mother, knowing that the questioner's mother suckled the suitor's older brothers?

It is permissible for your nephew to marry your daughter, unless there is a suckling relationship between them that establishes prohibition, such as if one of them was suckled by the other's mother or father, or if they both suckled from the same woman. The fact that his brothers were suckled by your mother does not prohibit her to him. The suckling that causes prohibition consists of five full sucklings.