The crossword solution for the term 'MOTHER OR SISTER' is NUN, which is a three-letter word. A NUN is a woman who has taken vows of poverty, chastity, and obedience in a religious order. Nuns are typically associated with the Catholic Church, although there are many other religious orders that include nuns. Nuns often dedicate their lives to prayer and service to others, and they are often seen as a symbol of motherhood or sisterhood.