Uncle Louie is the uncle of Howard Wolowitz and the brother or brother-in-law of either Sam or Debbie Wolowitz. Bernadette Rostenkowski-Wolowitz is his niece-in-law as Howard's wife. Halley Wolowitz is his great-niece as Howard and Bernadette's daughter.
He is mentioned in "The Lunar Excitation" when Howard tells Raj "I’ll show you a picture of my Uncle Louie in a bathing suit sometime." which leads to Howard appearing to ridicule his uncle who looked like he had female breasts.