Who are the two families that have had three generations of Oscar winners?

The only families with three generations of Oscar winners:
The Hustons - Walter Huston, Best Supporting Actor (1948); Walter's son John Huston as Best Director and Best Screenplay (1948); John's daughter Angelica Huston as Best Supporting Actress (1985).

The Coppollas - Carmine Coppola, Best Music (Original Dramatic Score) (1974); Carmine's son Francis Ford Coppola, Best Writing (1970, 1972, 1974), Best Director (1974) and Best Picture (1974); Francis' daughter Sophia Coppola, Best Writing (2003).
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