Just keep taking the most challenging courses at your high school. You're on track with your academics, try your best, and I'm sure you'll do well wherever you end up for school.
Your extracurriculars are good and I'm sure that over the next two years you'll participate in some other clubs and possibly get some leadership experience as well. It'll also be good to maybe do something that most kids don't do in high school...something that will set you apart from the rest of the applicants.
You have time! No worries. I'm sure you'll do the best you can with the application process, and having that connection is always a good thing...you're on the right track. A lot of it has to do with luck, so just remember you'll end up somewhere that you enjoy even if it's not Brown.