Is it forbidden for the questioner to marry his maternal aunt’s daughters, and is it permissible for him to see them unveiled, given that his older brother was nursed by the mother of these girls (his maternal aunt)?

Your maternal aunt's daughter and her sisters are not forbidden to you simply because your elder brother was nursed by your maternal aunt. They are non-mahram to you, and it is not permissible for you to see them in house clothes or to look at any part of their bodies. Rather, it is permissible to see them with complete hijab. Your brother, who was nursed by your maternal aunt, is considered her son and a sibling to all her children, male and female, through fosterage.