How did the story conclude in The Westing Game? Was it effective or not?

The story is a murder mystery about who killed Sam Westing, a paper products tycoon. Most of the plot revolves around a game his heirs play as part of his will. However, it turns out that Sam Westing is not actually dead. One of his heirs, young Turtle, discovers that he has been there all along masquerading as different people. She tells him she won the game and claims her prize, the inheritance. This is the story’s resolution.
Turtle, the heir who solves the puzzle, walks into Julian R. Eastman’s office at the end of the game when she realizes that he is really Sam Westing. She determined this partly because she kicked Barney Northrup, but Sandy MacSouthers had the bruise. Also, Sam Westing was a chess player, and Sandy was playing chess when he wasn’t supposed to know how.

She shouldn’t have kicked him (the Barney Northrup him). … He was smiling. He wasn’t angry with her, he was smiling.
“Hi, Sandy,” Turtle said. “I won!” (Ch. 27) 

Westing also pretended to be Julian R. Eastman, so he had all of the four directions covered with his aliases! He was Sam Westing, Sandy MacSouthers, Barney Northrup, and Julian R. Eastman.
Whether this is an effective or satisfying resolution or not is somewhat subjective. It is a twist ending, so it could be considered effective and satisfying in that respect. You may think it is less than satisfying because there wasn’t really a murder after all, so there was no murderer to identify. It’s a murder mystery without a murderer or a victim. However, that is all part of the fun.  
The ending is effective because it maintains the twists and turns of the story, which is quirky to the last. Sam Westing was an eccentric man. He faked his own murder, then rose from the dead and had his fun. In the end, it all worked out for the best.