I think you mean Jack Albertson
There was a Frank Alberston, although I don't know if he was Mabel's brother (my FAVORITE Mabel Albertson scene is in "What's Up, Doc?" as a man trips her up several times because he doesn't want her to get to the elevator.)
Biography of Frank Albertson from Leonard Maltin's Movie Encyclopedia:
"He's the one who didn't marry Donna Reed in It's a Wonderful Life (1946). One of Hollywood's perennial juveniles, Albertson appeared in dozens of films and plays during his 35-year tenure in show business. He began as a prop boy in the silent-film era; in his first major role, a young naval cadet in John Ford's Salute (1929), he displayed the youthful exuberance that would characterize most of his subsequent screen work. Perhaps best remembered as the naive young playwright ("Hail and farewell!") in Room Service (1938), although his later work included plum roles in Psycho (1960, as the boorish tycoon) and Bye Bye Birdie (1963, as the blustery mayor).
"OTHER FILMS INCLUDE: 1930: Born Reckless 1931: A Connecticut Yankee 1935: Alice Adams 1936: The Plainsman 1939: Bachelor Mother 1942: Wake Island 1947: The Hucksters 1958: The Last Hurrah"
Copyright © 1994 Leonard Maltin, used by arrangement with Signet, a division of Penguin Putnam, Inc.