A lot of people will tell her, "Take every opportunity you can! You'll only get these once!"
Don't do that. Trying to do everything will result in a lot of work, time, and stress. If she finds a few activities she truly enjoys, I think she should probably dedicate the most time to those. This way, there's more potential for holding leadership positions and being recognized in those clubs. And yes, personal enjoyment is important, but leadership and recognition are important in college admissions.
Also, time management is super important. Don't be like me and do your homework the morning it's due. Most of the time, procrastination results in stress and sloppy work (although I had some friends who did their last-minute work very well. If your sister is one of those people, then I guess she can do what works for her...?)
Several other people mentioned this already, but learn to talk to people. Not just students, but teachers and staff as well. She'll be able to make connections, friends, and get stronger letters of recommendation if she can be friendly with everyone.
I don't know if her school will offer AP classes, but I think they're a good investment if she's a decent test-taker. They're cheap (like $100) and will transfer into college credit at quite a few colleges and universities.
Write your college application essays in the summer between your junior and senior year. Don't wait until the fall because school and extracirriculars will take over. Ask teachers for letters of recommendation at the end of junior year. Apply for summer programs - there's free and expensive options.
Don't be afraid to cut ties with people who are not good friends. Life will be so much easier without the drama.
I don't think I have anything else to say right now, but I'll come back if I do :). I wish her luck in high school!