The term 'Big Sister' can be summarized by two crossword solutions: ABBESS (6 letters) and NUN (3 letters). An ABBESS is a female religious superior in certain Christian orders, while a NUN is a woman who has taken the vows of poverty, chastity, and obedience in a religious order. Both terms refer to a woman in a position of authority within a religious order, and can be used to describe a 'Big Sister' figure.