(coll.) mother's sister; maternal aunt

mother's eldest sister (older than one's mother); (coll.) (euphemism) Aunt Flo (i.e. menstrual period)

(strictly speaking) sister of one's maternal grandmother; (more broadly) grandaunt; sister of one's mother's mother or mother's father; (Tw) (old) husband's maternal aunt

husband of mother's sister; uncle

(coll.) wife's younger sister; sister-in-law (term not used to directly address her)