Character Analysis
Alongside Waymarsh and Mrs. Newsome, Sarah is the main antagonist in this novel. She's like some sort of demon who Mrs. Newsome sends out into the world to carry out her orders, and she's really the closest thing we ever get to hearing Mrs. Newsome directly. In a word, this lady's scary.
Strether is downright afraid of Sarah because, that's right, she's the aggressive, bullying type. She calls Chad's transformation "hideous" (10.3.30) and feels that Madame de Vionnet is one of the most disgusting women she has ever met. Ouch.
Further, Sarah refuses to believe that anyone in the world could ever be nearly as good as her mother. Looks like someone never had a rebellion phase in high school. When Strether asks if she might learn something from Madame de Vionnet, she answers, "A 'revelation'—to me: I've come to such a woman [Madame de Vionnet] for a revelation […] when the most distinguished woman we shall either of us have seen in this world sits there insulted, in her loneliness, by your incredible comparison!" (10.3.20). It's more or less at this point in the book that Strether realizes he'll never get Sarah to see anything beyond her rigid family prejudices.
As much as Sarah's strict worldview suggests she's got a lot of confidence, the woman is actually very insecure about being a big fish in the small pond of Woollett, Massachusetts—which is like, a really small Pond compared to Paris. At the end of the day, she's just not as big a deal as she thinks she is.
For example, when Strether tries to push her to see how good Chad's transformation in Paris really is, she answers, "I leave you to flatter yourself […] that what you speak of is what you've beautifully done. When a thing has been already described in such a lovely way […] Do you consider her even an apology for a decent woman?" (10.3.16). Again, she goes back to attacking both Strether and Madame de Vionnet, although she has absolutely nothing to base this attack on. She never backs up her judgments with any actual arguments. She just keeps saying over and over that her mother is the greatest person on Earth and everyone else is bad.
There's not really any arguing to be done with that sort of person.
Like Waymarsh and Mrs. Newsome, Sarah also hates the fact that people are out having fun and enjoying life while she's sitting in Woollett, Massachusetts (or for part of the book, in the Blah Hotel, Paris) trying to be a calm and respectable lady. As Strether concludes late in the book, Sarah isn't truly furious at him and Chad because she thinks they've changed for the worse: she's furious because she recognizes that they've changed for the better.
But she wants to drag them back to Woollett out of spite, because how dare they enjoy themselves instead of obeying Mrs. Newsome's every word? Not a healthy way to live, if you ask us.
Sarah's Timeline