What is the big idea the author is addressing?

This is a thematic question. I might consider Things and People Are Often Not as They Seem. It starts with the opening sentences: the luxury apartment building is not a regular rental property—it is meant to lure only specific renters; the rental agent, Barney Northrup, is not a real agent at all—the reader is told that he isn't even a real person. By the end of the first chapter, we're ready to become suspicious of everyone. Some of the characters don't even know everything about themselves; they're surprised to find that they've been named as heirs. A will is read, but it's actually instructions for a game. It seems that the will and the game it starts are meant to destroy one person and drive others apart, but it helps people break through barriers, find hope, and become closer; this theme continues until the final page.

Throughout the Westing Game we're reminded not to judge people too quickly. Some who appear sinister turn out to be kind. The beautiful, submissive Angela turns out to be a rebel and a bomber! The heirs appear to have lost the fortune, but they have gained new friends, refreshed relationships, and skills, contacts, and support that assure each one a bright future.