It's nice how the harp part is so audible - I love that little riff
I think the basic "problem" with Frank Sinatra, Jr. is that the sense of entitlement (due to being one of Frank Sinatra's kids) he has doesn't work for him like it does for Nancy. Nancy is Sinatra's little girl, and there was the song about her and everything, so she was able to carve out a little niche for herself in pop culture, as well as in the entertainment industry. He, on the other hand, was unfortunately destined to forever be but a shadow of his legendary father. That's life.
Incidentally, "Somethin' Stupid" was written by C. Carson Parks, brother of frequent Brian Wilson collaborator, Van Dyke Parks, in 1967. C. Carson Parks recorded a version of the song with his wife as a duo called "Carson and Gaile" that same year.