How do I convince my brother to THINK HARD before he marries a white woman, without sounding racist?

While I believe your intentions are pure, your opinion is based on the assumption that your brother is going for a "trophy" white woman. This may or may not be true. Successful men do it all the time, but it doesn't mean they won't have a good marriage. I'm afraid there really isn't a way for your to express your opinion to him without looking like your a bit prejudiced and even if you are right you're still going to offend him by indirectly calling him shallow. I would leave race out of the picture all together and just ask him if he's sure this is the right woman. You may have a lot of valid points that don't even have anything to do with race. Probably not much you can do about it anyway. Men can be stubborn and often fall in love with what they want regardless of the reason for why they want it.