I like to always work in at least two families. That way, if one is unavailable, I still have current hours. Unavailable can mean a vacation, a hospitalization, they moved away, or my client died.
Unavailable can also mean that the parents want a different nurse. It happens. It's happened to me three times, although one of those times, it wasn't against me; it was because the parents wanted a specific nurse, and we couldn't both work at the same time.