Who is the thief in The Westing Game?

Madame Hoo, the immigrant wife of the owner of the Chinese restaurant at the top of Sunset Towers, has been stealing trinkets to sell in order to go back to China.  She speaks very little English, and has been lonely and isolated since she came to the U.S. to become Mr. Hoo's second wife.  She has been working in the kitchen of the restaurant, and has no friends outside of her husband and stepson.  With no one to confide in, Madame Hoo has become desperate to return to China.  Hoo's restaurant has not been doing well, so she miguidedly believes that she must steal things to sell in order to get money to go back to her homeland.

She reveals herself as the thief in a touching scene in Chapter 26:

     A trembling Madame Hoo stood before the judge.  "For to go to China," she said timidly, setting a scarf-tied bundle on the desk.  Weeping softly, the thief shuffled back to her seat.
     The judge unknotted the scarf and let the flowered silk float down around the booty; her father's railroad watch, a pearl necklace, cuff links, a pin and earrings set, a clock.  (Grace Wexler's silver cross never did turn up.)
     "My pearls," Flora Baumbach exclaimed with delight.  "Wherever did you find them, Madame Hoo?  I'm so grateful." 
     Madame Hoo did not understand why the round little lady was smiling at her.  Cautiously she peered through her fingers.  Oh!  The other people did not smile.  They know she is bad.  And Mr. Hoo, his anger is drowned in shame. 
     "Perhaps stealing is not considered stealing in China," Sydelle Pulaski said in a clumsy gesture of kindness. (168)

Madame Hoo confesses because she believes that she will be found out.  She is not prosecuted for her petty crimes, and the rest of the heirs, though embarassed, feel pity for her. 

Source: Raskin, Ellen.  The Westing Game.  New York: Avon Books, 1978. 

In The Westing Game, who is the thief and what motivates them?

The people living in the Sunset Towers realize early in The Westing Game that little items are disappearing or being stolen. Several of the items have both monetary and sentimental value. Ultimately, it is Sydelle Pulaski's little notebook that proves to be a theft of larger proportions.

Sydelle is the only tenant who has the foresight to take notes during the reading of the will. The other players realize the value of her notes because the will offers small clues to the game and the rules. Sydelle realizes that her notes give her team a distinct advantage, so she hides the notebook in her room. Unfortunately, the notebook is stolen.

Sydelle Pulaski stared at the bare wicker bottom, then sank to the rim of the bathtub, shaking her head in disbelief. Someone in Sunset Towers had stolen the shorthand notebook.

The notebook reappears in chapter 11 when Mr. Hoo finds it on a table in his restaurant. Sydelle accuses Mr. Hoo of being the thief, and he flatly denies the accusation.

“I did not steal your notebook,” the indignant Hoo explained. “I found it on a table in my restaurant this morning."

This little exchange plants the seed that Mr. Hoo might have something to do with the thief, but there won't be confirmation of that until chapter 26. It turns out that Mr. Hoo is married to the thief. His wife, Madame Hoo, has been stealing things to sell in order to get enough money to return to China. She doesn't speak much English, doesn't have any close friends, and is working in a restaurant owned by her husband. She is feeling lonely and homesick, and the restaurant isn't doing well enough to pay for the trip, so Madame Hoo turns to stealing.

“Yes, please.” A trembling Madame Hoo stood before the
judge. “For to go to China,” she said timidly, setting a scarftied bundle on the desk. Weeping softly, the thief shuffled back to her seat.

The judge unknotted the scarf and let the flowered silk
float down around the booty: her father’s railroad watch, a
pearl necklace, cuff links, a pin and earrings set, a clock. (Grace Wexler’s silver cross never did turn up.)