Is it permissible for my sister to marry her brother, given that my maternal aunt breastfed me and her daughter only once?

Your sister has no relation to this suckling. She may marry whomever she wishes from her maternal aunt's sons. Whether the suckling you received from your maternal aunt was complete or incomplete, as long as it was less than five known sucklings, her daughters are not forbidden to you. Therefore, there is no harm for you to marry whomever you wish from your maternal aunt's daughters, according to the preponderant opinion.