Is it permissible to marry the daughter of one's maternal aunt who was breastfed by the questioner's mother, along with his younger brother, unknowingly and unintentionally?

If it is confirmed that your maternal aunt's daughter was suckled by your mother, even once, then she is your sister through suckling, and it is forbidden for you to marry her. If the marriage has already taken place out of ignorance or forgetfulness, then the marriage is void and must be annulled immediately. The children are attributed to their father due to the absence of the prescribed punishment (hadd) in cases of doubt.