Katharine McPhee and David Foster face a "horrible family tragedy."
Their young son's nanny passed away unexpectedly.
McPhee withdrew from joint tour in Asia, returning to the US.
McPhee en route to California; details about nanny's death undisclosed.
Unclear if their 2-year-old son was on tour with them.
McPhee's Instagram post expressed regret for missing tour shows.
No immediate response from reps for McPhee and Foster.
Tragic situation interrupts "David Foster and Friends" tour in Asia.