Is it permissible for the questioner to uncover before her maternal uncle’s sons, from whom her brother suckled, and what is the ruling concerning the questioner’s sisters in relation to them?

If the suckling is established by five or more feedings, and the suckling child is within the two years (of age), your brother becomes the son of your maternal uncle and his wife through suckling. Their children become his siblings, and your maternal uncle's brothers become his paternal uncles, and his sisters become his paternal aunts. The suckling woman's brothers become his maternal uncles, and her sisters become his maternal aunts, due to the Prophet's saying, peace and blessings be upon him: "What is prohibited by lineage is prohibited by suckling." As for you and your sisters, you are not related by this suckling, and it is not permissible for you to uncover your hijab to your maternal uncle's sons because they are not mahram to you.