My brother is marrying American. What to do?

What's wrong with marrying an American? Although I'm Australian and see the problem, it's not the end of the world. Is it the case you think she is not good enough for him? Blood is thicker than water, and love is also blind so he may be blind to toxic personality traits that your seeing and he can't. You can always have a sisterly chat and if the marriage goes sour, you can say, "I told you so". Nothing is forever these days.