Hello Dana and welcome to the forum!
Yes, braces can widen your smile when you are an adult (by tipping the teeth out, or by putting tipped in teeth upright), so that your smile looks wider. But braces cannot expand your palate as an adult, because your palate is fused. However for extreme cases, there is a surgery (SARPE) to open the suture of your palate, so that expansion can still happen.
I also have a narrow palate and a bit of a crossbite (do you also have a crossbite: is your upper jaw smaller than your lower jaw?) and braces have definitely broadened my smile, but for the best results I would have needed surgery to expand my palate (which I declined,at least for now).
I wish you very good luck on your appointment on Monday!