A: Why isn’t Mary on good terms with her elder sister?

B: Because she resented always playing second fiddle to her sister.

Note: This saying refers to a person who plays a supporting or subservient role. It derives from the orchestral role of principal second violin. This person leads the group of second violins who play a supportive role musically to the first violins which usually play the melody. Early uses of the expression in print were found in early 19th century London.