Several years ago I read a SF book about a young man engaged to a girl whose father ran a big corporation. Visiting the stronghold the young man met the girl's kid sister and made friends with her. The father sends him off planet for several years and when he finally sees them again in space he doesn't like the older girl but the kid sister is grown up and he likes her. Part of the story involves his writing music and the kid sister getting involved in his music.
Variable Star by Spider Robinson.
Star is a 2006 novel written by Spider Robinson based on the surviving seven pages of an eight-page 1955 novel outline by the late Robert A. Heinlein. The book is set in a divergent offshoot of Heinlein's Future History and contains many references to works by Heinlein and other authors. It describes the coming of age of a young musician who signs on to the crew of a starship as a way of escaping from a failed romance.