Probably true. From imdb.com :

- The decision to cast Marlon Brando was hotly contested, largely by Frank Sinatra, who wanted the part of Sky Masterson himself. Later in his career, he made Sky's big number "Luck Be A Lady" part of his stage act.

- Frank Sinatra loathed the non-singing Marlon Brando for getting the starring role in Guys and Dolls, whereas Sinatra got a lesser part, and his nickname for the sometimes barely coherent Brando was Mumbles.

However, to Sinatra's credit...in the same soundbite in which he tells the 'Guys & Dolls' singing story, he does proclaim Brando one of the great actors of the day. To which the audience is heard applauding with agreement.

I guess despite personal feelings, he wasn't afraid to openly respect talent where respect was due.

Apple