The term 'SISTER OR MOTHER' can be summarized by the crossword solution 'NUN', which is a three-letter word. A NUN is a woman who has taken religious vows and is a member of a religious order, typically one devoted to teaching, nursing, or other charitable work. Nuns are often associated with the Catholic Church, but there are also nuns in other Christian denominations, as well as in Buddhism and Hinduism.