Is the son of my mother's husband, whom my mother breastfed along with my brother, considered a mahram to me, even though he is younger than me?

Breastfeeding prohibits what lineage prohibits. So, whoever is breastfed by a woman becomes her son and a brother to all her children, whether he was breastfed with them or not, and whether they were born before or after him. If your mother breastfed her husband's son five full and satisfying breastfeeds or more, then he has become your brother through suckling. However, if the breastfeeding was less than five feeds, then prohibition is not established by that, according to the preponderant opinion.