(Don't let the "Mrs." fool you: Mrs. Western is Squire Western's sister, not his wife. She goes by "Mrs." because she is an older lady, but she is not married.)
In a lot of ways, Mrs. Western seems like what Squire Western would be if he had been born a woman. She is incredibly arrogant and sure of herself. And like Squire Western, she has absolutely no interest in Sophia's protests that she doesn't want to marry Mr. Blifil. Even more horrifying, she refuses to listen to Sophia's objections to Lord Fellamar, even after Sophia tells her that Lord Fellamar tried to assault her at Lady Bellaston's house.
Mrs. Western's stubbornness may not be as rough or as obviously violent as Squire Western's, but the two siblings are obviously cut from the same bullying cloth. In fact, the narrator even notes that Mrs. Western has a "masculine person" (6.2.2) over six feet tall, and that her manners and appearance are pretty manly. So Mrs. Western even looks like her brother.
So, if this brother and sister are two peas in a particularly obnoxious pod, why does the narrator include both Westerns? What particular role does Mrs. Western play?
One reason we think the narrator includes Mrs. Western in this book is that she takes part in lots of hilarious (or at least, so Fielding seems to think) conversations with her brother. There are tons of squabbles between the two siblings over the course of the novel. Mrs. Western will say something random and historically inaccurate about politics (since she prides herself on her learning) and Squire Western will fire back with something nasty about Hanoverian round-heads (about which, see our list of "Allusions").
It's not just politics, though. The two of them each blame the other for how Sophia has turned out (ungrateful and unwilling to marry Mr. Blifil). And they both use gender against each other. Mrs. Western keeps making all of these sarcastic remarks about "weak women" (6.2.5) (obviously meaning that women are anything but weak). And Squire Western complains whenever the two argue that Mrs. Western is a "b—" (7.5.2) to him. (We talk more about Mrs. Western's femininity in our "Quotes and Thoughts" section under the theme "Gender.")
These conversations may not seem knee-slappingly funny to us, now that we are so far away from the social and historical issues these two keep throwing at each other. But Fielding is clearly playing these two stubborn old coots for laughs. They get so fired up about abstract issues that neither of them truly understand. And it causes them to miss what's most obvious to everyone: they are both really, really similar people.
Mrs. Western's other truly important character trait is that she is vain as all get-out. We see this vanity over and over again in the novel. So, for example, while Mr. Fitzpatrick is wooing her niece Harriet right under Mrs. Western's nose, she never notices. She is so certain that Mr. Fitzpatrick wants to marry her that she never spots her niece's terrible romantic plans.
If Mrs. Western had been a little less vain and a little more perceptive, she might have saved Mrs. Fitzpatrick a lot of heartache. (Though who knows—Mrs. Fitzpatrick seems pretty stubborn and decisive on her own; maybe Mrs. Western couldn't have stopped her niece from eloping with Mr. Fitzpatrick, even she had noticed their romance).
By contrast, Sophia is smart enough (and manipulative enough—Sophia definitely has hidden depths) to use Mrs. Western's vanity against her. While they are staying together in London, she flatters Mrs. Western by reminding her of all of those suitors Mrs. Western says she used to have. But Mrs. Western refused every one of their many proposals (or so she says).
So why should Sophia get married against her will, when she has Mrs. Western's excellent example in front of her, encouraging her to follow her own path? This logic only soothes Mrs. Western for a time, but it temporarily saves Sophia from having to spend more time with Lord Fellamar—definitely a positive result.
Still, overall, Mrs. Western's vanity is incredibly destructive. It is because she is so vain that she just won't listen to Sophia when Sophia tries to tell her the truth about her feelings for Mr. Blifil and, later, Lord Fellamar. As soon as Mrs. Western hears that a lord wants to wed her niece, she decides Sophia absolutely must marry him for the sake of the Western family honor.
As with Squire Western's bad temper, the novel never truly punishes Mrs. Western's vanity. After Sophia marries Tom, she and her aunt make up, and it's as though nothing ever happened between them. Mrs. Western tried to bully Sophia into marrying a man who tried to rape her. That's not something that we think anyone should have to forgive. But the novel just lets it go in the name of family unity.