This woman will catch you by surprise.
We don't know much of where she comes from, but we know a lot about the caliber of woman she is. Initially she seems to be a pushover, always going silent when her husband Eddie flies into a tantrum and accepting that her husband has mistresses. But do not underestimate her.
She is of the Tungs, a politically connected family, and has raised three "very studious, well-behaved children" (1.6.9). A number of times in the novel we see her defend her children from Eddie's insane requests and attacks, but she never does quite enough to get him to apologize or stop doing it in the future.
Finally, before the family leaves for the biggest event of the wedding, Eddie threatens to hit their six-year-old son. She announces that she and the children will not be riding with Eddie to the event, thwarting all his plans for a perfect entrance (which he was upset about in the first place) and says, "Oh, don't worry about me […] I was born a Tung, and I have nothing to prove to anyone" (3.6.89).
This is a small step, but a significant one. And why is it important? It's a signal to many members of the family that a) she's not taking it anymore and b) they don't need to take it anymore. Just because they weren't born a Young, Shang, or T'sien, they should not be relegated to terrible treatment.