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.