In recognition of National Sibling Day 2016, I investigated the use of variants of the words brother and sister in songs from the Billboard year-end Top Ten charts (1951-2015). In all, the terms occur 41 times in 19 different songs. The frequency of variants for the two terms is almost identical, with both brother(s) and sis(ter) occurring 20 times in the collection, and the tiebreaker, brotherhood, occurring once (in Wiz Khalifa's 2015 song "See You Soon"). 

Whether the use of brother or sister in a song is in reference to an actual sibling is not always transparent. In Bobbie Gentry's 1967 song "Ode to Billie Joe," there are several uses of brother, and while several are apparently in reference to the narrator's male sibling, one use is in reference to a "nice, young preacher, Brother Taylor." Brother also appears in several songs as a term suggesting a close bond between friends or as a generic term for male, and is most frequently used in this way by African American artists, such as Bill Withers (in 1972's "Lean on Me"), Bobby Brown (in 1988's "My Prerogative"), Sir Mix-A-Lot (in 1992's "Baby Got Back"), Puff Daddy and Mase's (in 1997's "Can't Nobody Hold Me Down") and Beyonce (in 2009's "Single Ladies"). While it is not unambiguously used as a religious title (the jury is still out on its use by The Highwaymen [in 1961's "Michael"]), the term soul sister(s) appears in "Baby Got Back" and several times in Train's 2009 hit "Hey Soul Sister." These occurrences function in ways similar to brother above.

Interestingly, whereas in none of the uses of brother is there any indication of whether the brother in question is younger or older, several times there is an indication in the collection that the sister being referred to is younger, as in the utterance "my little sis" by Chubby Checker in "The Twist" (1960, 1962) and "my little baby sister" in "The Loco-Motion" by Little Eva in 1962 and another version by Grand Funk Railroad in 1974.

And to leave off, one of my favorite songs that includes the terms brother that never cracked the year-end Top Ten (ending the year 1971 at #21 on the Billboard charts):