With a GPA that high, and those ECs, AND a minority, I can see him being accepted to that list of schools. Cousins are not primary legacies, but can give a bit of a boost. Some colleges consider a sibling to be a legacy, so you can check that for WashU (MIT does not consider legacies at all).
If you are an underrepresented minority, your acceptance rates will be higher to some of these colleges.
I still think that you should look for colleges at which you will find an atmosphere in which you feel more comfortable. In any case, I think that you should work hard during your Junior year, and see where you are standing next summer, by way of GPA and SAT/ACT scores.
You should also sit down and really think what you want from college, and what would make you happy in college. What sort of things do you like doing? What sort of classes do you like (both topic and how they are taught)? What sort of people are “your people”?
Your college goal should not be “admission to a prestigious college”. It should be “getting the most out of college”. When you want to get to the airport, do you care whether the Lyft is a Porche or a Toyota? You want the ride to be safe, comfortable, and get you to the airport on time. The point of college is to get you to where you want to be after college. So you need to find which college will get you where you need to be, academically, professionally, socially, emotionally, and financially. Those are the things that you should consider, not the Brand Name of the college.