Is it permissible for a mother to swear an oath not to attend her son's wedding if he marries a specific girl, and is expiation (kaffarah) required for this oath?

It would have been better for your mother not to resort to an oath, nor to repeating it. After she swore, it is best for her to offer a single expiation as long as she only swore about one thing. Furthermore, insistence on an oath and adhering to its requirements, especially when it leads to hardship, difficulty, or the abandonment of benefit, makes it more deserving of being broken and expiation offered. Your mother should not object to her son marrying that girl on the pretext that she does not hold the nationality, as long as she is pleasing in her religion and character. Similarly, the son must obey his mother's command if she insists on him not marrying this girl, provided there are others available.