The Great Gatsby, sterner

Could you please tell me the meaning of "sterner" in this passage extracted from the chapter 4 of The Great Gatsby?

Benny McClenahan arrived always with four girls. They were never quite the same ones in physical person, but they were so identical one with another that it inevitably seemed they had been there before. I have forgotten their names—Jaqueline, I think, or else Consuela, or Gloria or Judy or June, and their last names were either the melodious names of flowers and months or the sterner ones of the great American capitalists whose cousins, if pressed, they would confess themselves to be.

Thank you;