This is a good question. As you probably know, the thesis statement is the most important part of an essay. In this case, you should probably start with an understanding of what the Yin and Yang concept is, so that you can focus on appropriate examples of this in your paper.
In the most basic way, the yin and yang refers to how polar opposites or things that seem very distant are actually connected and even dependent on each other. The difficult of seeing this concept is, of course, that they are so different and on surface it might appear that there is nothing in common. However, under closer scrutiny, there is interconnectedness. So, you would want to look for examples of these opposites in the novel. Let me start you off with a few possible leads.
First, June and Waverly are so different in the novel, but there are related in many ways. Exploration of their personalities may show the yin and yang concept. Second, the old and new is also another contrast. The old ways of life in China and the new life in America are played off of each other.