What is the Islamic legal ruling for a wife suckling her brother, and for the wife's mother suckling the questioner's son? And who are the son's unmarriageable kin (mahrams), and does the questioner become unmarriageable to anyone?

If your wife's mother breastfeeds your son five known feedings within the two years, he becomes a brother by suckling to the grandmother's sons and daughters. Consequently, the daughters of the grandmother's sons become mahram to him because he is their paternal uncle by suckling, and the daughters of her daughters become mahram to him because he is their maternal uncle by suckling, due to the saying of the Prophet, peace and blessings be upon him: "Suckling prohibits what lineage prohibits." However, he does not become mahram to any of them.

And if your wife's brother is breastfed by her mother five known feedings within the two years, he becomes her son by suckling, and you become his father by suckling; because prohibition is established through the milk of the male.