NGS Style Manual‎ > ‎- B -‎ > ‎

brother, sister

Capitalize when preceding a personal name in religious reference or when part of the name of a religious order:
            Brother John, the brother
            Sister Mary, the sister
            Ursuline Sisters, the sisters

Otherwise lowercase:
            the Wright brothers
            the brothers Grimm
            the Brontë sisters

Except in company or trade names:
            Warner Bros.
            Smith Brothers
            Ringling Bros. and Barnum & Bailey Circus (show)
            Ringling Brothers—Barnum & Bailey Combined Shows Inc. (company)

And: the Seven Sisters (Pleiades)

See also FAMILY RELATIONSHIPS.