I would say the best thing you can do in high school (and undergraduate studies) with your eye toward the law is focus on improving your writing skills. The ability to write intelligently is a powerful tool to have regardless of what profession you finally end up in.
With law, it’s about your ability to focus on the issue at hand and deliver your message using the facts to the best of your client’s position.
I think one of the best undergraduate classes I took was one that had to do with logical thinking. It sounds simple, but it’s not as easy as it sounds. I’ve heard many times “This person should be an attorney because he/she is always arguing.” That’s not an attorney. That’s someone who will need an attorney. I’d rather hear “This person should be an attorney because he/she always takes the time to take in all the facts, thinks logically, and comes up with a reasoned response that makes sense.” That sounds like an attorney. So working on that skill will help you tremendously.
I hope that helps. I wish you luck in whatever you decide to do!!
Paul V.’s Answer
If you're not familiar with the Occupational Outlook Handbook published online by the Bureau of Labor Statistics (bls.gov/ooh), I would strongly recommend taking a look! In addition to finding out what an attorney (or any job existing in the US) does, it also includes information about the education needed, the potential salary ranges, and the outlook for the next 10 years. I think that both you, and your sister, could benefit from taking a look at this resource as you navigate through the vast opportunities out there.
Logical reasoning and thinking
Ability to present arguments well in writing and/or speaking
I agree with several of the other answers given - communication skills (writing, speaking), investigative skills (researching, reading, analyzing), advocacy/persuasion skills (being able to construct arguments to advocate a position and persuade others to accept or consider your position) - are very important for practicing law.
I would emphasize another skill that I think underlies all of these - the ability and willingness to think. I recall on my first day of law school one of my professors stated that he was not going to teach us a particular area of law but how to think, and we (the students) would teach ourselves the law. In my personal experience, I've found the professor's approach was correct. A critical skill I use (or endeavor to use!) is the ability and willingness to take any issue, gather and analyze information about the issue to understand it, and then reach reasoned conclusions about the issue based on a particular set of facts. Thankfully, that skill can be and is taught in law school.
If your sister would enjoy thinking, among the other skills mentioned in the answers, then I think law may be a good career choice for her.