Is it forbidden to marry the daughters of maternal aunts and uncles, and is it permissible for them to remove their niqab in front of me, and am I considered a mahram to them for 'Umrah, given that my maternal grandmother breastfed me five full feedings?

If the grandmother breastfeeds her grandson five times or more, the grandson becomes her son through suckling (radāʿa), and he becomes a maternal uncle to his maternal aunts' daughters, and a paternal uncle to his maternal uncles' daughters. Thus, it becomes forbidden for him to marry his maternal aunts' daughters and his maternal uncles' daughters. They are also permitted to uncover their faces in his presence, just as he is permitted to be their maḥram for lawful travel.