Who is the answer to the Sam Westing's will?

Sam Westing was never killed. The dead body in the bed was simply a wax dummy. In the will it says "The one who finds the windfall will be the one that finds the. (Section three ends, and section four starts below this.)

Fourth"

What this means is that windfall (the directions the wind blows, North South, East, West) and that Same Westing is actually 4 different people. Barney Northrup, Sam Westing, Sandy McSouthers and Julian R. Eastman (If you want to be technical, he was also Samuel "Windy" Windkloppel because that was his name before he changed it to Sam Westing). Turtle is the only one that finds out the 4 people that Sam Westing was, and she goes to meet Julian R. Eastman (no, Turtle does not receive $200M for winning the game) and she and Mr. Eastman become friends. Then, at age 85, Mr. Eastman dies of old age.