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.