I'm not sure I'm really making any progress on this, but wonder if it could possibly be to do with cousins?
Giles Swayne is a cousin of Elizabeth Maconchy
Berlioz - Béatrice et Bénédict: Héro and Béatrice are cousins
Janáček - Jenůfa: teva and Laca are both in love with their cousin Jenůfa
Mozart - Carl Maria von Weber was the cousin of Mozart's wife, Constanze
Perhaps Zappa had a famous cousin or wrote a song about one?
Shostakovich? No famous cousin that I can locate, but he does have a famous son, so is he the odd one out?!