What is the ruling on the marriage of my mother's son to the daughter of my maternal uncle, who was nursed with my sister by my grandmother?

It is permissible for a mother to marry one of her sons—who was not suckled by their maternal grandmother (the children's grandmother)—to one of the daughters of the children's maternal uncle. There is no prohibitory kinship (mahramiyya) between siblings and the maternal uncle's children, except with the daughter whom the grandmother suckled. Their relationship to this particular daughter would be as the son or daughter of their suckling sister's brother, and the prohibitory kinship of a sister's brother's son is not mentioned in the Qur'an or Sunnah (definition).