Is a brother through رضاعة (milk-kinship) a محرم (unmarriageable kin) to my sisters, and are his brothers محارم to me? And is the father of my sister's son, who was nursed by my mother along with me, a محرم to me?

Suckling makes unlawful what lineage makes unlawful, and it spreads the sacred prohibition (hurmah) between the-suckled, his parents, and his siblings by suckling. The ruling of suckling is specific to the-suckled and does not extend to his brothers or sisters. Therefore, your brother's suckling brothers are not your mahrams, and your brother's suckling father is not your mahram (definition) unless you have suckled from his wife's milk. As for his brothers, if they are the sons of your sister, then they are your mahrams because you are their maternal aunt by lineage.