Does the son of my wife's son become a brother to my wife through رضاعة (رضاع, suckling), and محرم (unmarriageable kin) to my daughter, making it permissible for her to uncover in front of him, if my wife's mother (his grandmother) breastfed him more than five times?

If the boy becomes a brother to your wife through suckling, then he is an uncle to your children from this wife. Therefore, it is permissible for your daughter from this wife to show him what she shows to her mahrams. The evidence for this is the Prophet, peace and blessings be upon him, said: "What is forbidden by lineage is also forbidden by suckling." And a sister's daughter is a forbidden relative by lineage, so a sister's daughter by suckling is also forbidden. The suckling that makes one a mahram is five or more sucklings within the first two years of life.