Ellen Rankin, in The Westing Game, offers several lessons to the reader.
She uses the novel to explain the negative consequences of being wealthy or having an overabundance of money, and of wanting money. Some of the characters' money represents freedom, or the ability to get an education but almost all of the characters are willing to lower their moral standards to get money.
The question of who and what a family is comprised of also arises. In the story, you see the development of the idea that there are two types of family: the family you are born into, and the circle of people who support you. Flora becomes like a mother to Turtle because Turtle does not have a good relationship with her own mother.
In addition, the author develops the theme of identity. Who is Sam Westing? That is the $200 million dollar question. Throughout the book, the participants in the Westing Game determine who they really are. They refine themselves to create their true identities.