A brother or sister is a sibling, and can be referred to by several different terms. The crossword solutions for this term are 'nieces' (6 letters), 'AMEN' (4 letters), 'niece' (5 letters), and 'SIB' (3 letters). 'Nieces' and 'niece' refer to a female sibling, while 'SIB' is a gender-neutral term. 'AMEN' is not directly related to the term, but is often used as an expression of agreement or understanding.