Is it permissible to marry my maternal aunt's daughter if my grandmother – my mother's mother – breastfed me once and also breastfed her older brother?

A single suckling does not prohibit marriage. Therefore, your cousin is not forbidden to you. The majority of scholars hold that the testimony of the wet nurse alone does not establish prohibition, unlike the Hanbalis, whose view is more widely accepted. If you were to refrain from marrying her, it would be more cautious. As for your brother suckling from your shared grandmother, this does not affect your marriage to his sister.