Is your brother's son considered your nephew through breastfeeding, and do you become his paternal aunt and his mahram after your brother's wife breastfed him five full, satisfying breastfeeds, or does he remain a stranger to you and not one of your mahrams?

If the child has suckled from your brother's wife five times or more, then he becomes one of your mahrams; thus you become his paternal aunt through suckling, due to the Prophet’s (peace and blessings of Allah be upon him) saying: “Suckling prohibits what lineage prohibits.”