Is it permissible for me to marry my maternal aunt's daughter, knowing that her elder brother was nursed by my grandmother, and I also was nursed by my grandmother?

The prohibition due to fosterage is established like that due to lineage; thus, what is prohibited by lineage is also prohibited by fosterage. The suckled child becomes a son to the wet nurse and her husband, and the prohibition is specific to the nursed child himself, not extending to his brothers and sisters. Therefore, your grandmother's suckling of your maternal aunt's son is specific to him alone and does not include his sister. Hence, it is permissible for you to marry your maternal aunt's son's sister, unless there is another legitimate fosterage between you, knowing that the prohibiting fosterage must consist of five sucklings.