The answer is: no one! Sam Westing died of old age, although he lived the last years of his life under a pseudonym (and in disguise, for part of the time, as Sandy McSouthers the doorman.) Turtle Wexler is the one who discovers that Sam Westing is not really dead, and has been among the "heirs" all this time masquerading as Sandy. After the "game" is ended, Turtle, who becomes a business tycoon, befriends the aging Mr. Westing. He actually had lived under four different names
The heir who wins the windfall will be the one who finds the fourth. It was so simple once you knew what you were looking for. Sam Westing, Barney Northrup, Sandy McSouthers (west, north, south). Now she was on her way to meet the fourth identity of Windy Windkloppel. She could probaby have figured out the address, too, instead of looking it up in the Westingtown phone book -- there it was, number four Sunrise Lane. (174-5)
Turtle never tells anyone that Sam Westing is alive, but she visits him weekly until he dies peacefully of old age. The corpse she had found in the beginning of the book was a wax copy, and Sam Westing's personal doctor had been in on the conspiracy to make people believe that Sam was actually dead. The whole ruse was created by Sam to bring his "heirs" together and right some past wrongs. The game actually helped all of the heirs realize their true potentials, and take the proper direction in their lives.
Source: Raskin, Ellen. The Westing Game. New York: Avon Books, 1978.