The easiest way to determining the falling action of a plot line, is to first identify the conflict and the climax. Remember, the falling action usually immediately follows the climax and often contains the resolution to the conflict.
In this mystery, the most prominent conflict is, of course, who murdered Sam Westing, and which pair of heirs will win Westing's fortune? The climax occurs both during and just after Turtle's mock trial, where she is the only one to fully solve the Westing mystery. The final page of the will is read where the heirs are instructed to share the title to Sunset Towers, and all the game money is left to Westing's former wife, Crow. Then in a fantastical fireworks display, the Westing house burns to the ground. There is no mentions of the million dollar fortune and the heirs assume no one won.
The falling action includes Turtle revealing to Julian Eastman the full answer that the four "winds" of Windy Windklopple are each Westing, the fourth of course being Julian Eastman himself. She continues her relationship and playing chess with "Sandy" (also Westing) and never tells anyone his secret. Each of the characters moves on to have positive and fulfilling futures. Turtle remains close with Westing until the day he dies (as Julian Eastman) and never reveals to anyone, even her husband, the fullness of the mystery solved.