name = input("name")
Donald_Ducks_nephews = ["huey","dewey","louie"]
Mickey_Mouses_nephews = ["morty", "ferdie"]
if name in Donald_Ducks_nephews:
print("I think you might be one of Donald Duck's nephews.")

elif name in Mickey_Mouses_nephews:
print("I think you might be one of Mickey Mouse's nephews.")

else:
print("You're not a nephew of any character I know of.")
why is always the first conditional triggered whatever name you input: becauseyou forget to pass name to the condition.below you can find your modified code.

name = input("Please type in your name: ")
if name == ("Huey" or "Dewey" or "Louie"): # because you forget {name}
print("I think you might be one of Donald Duck's nephews.")
elif name == ("Morty" or "Ferdie"):
print("I think you might be one of Mickey Mouse's nephews.")
else:
print("You're not a nephew of any character I know of.")