Hi Jess, thanks for your question. What I would say to you is maybe just don't be in such a hurry to grow up. Enjoy the privilege of being young for a little bit longer. If it is any consolation I was in my late 20's before I really knew what I wanted, it's OK. Your not supposed to know what you want at 15, being confused is very normal!
By comparing yourself to your sister your being very hard on yourself, if you must compare, compare yourself to her when she was 15, not 18, you may far outshine her by the time your 18. You have the whole of high school to catch up. You don't know what will happen in those years.
Another thing to remember is that you don't have to be great on paper to be a great person and a great friend. Does a straight "A" student make a better friend than a "C" student? Is somebody who knows what they want to do in life more fun than somebody who doesn't? Of course not!! You can be fun, outgoing and popular without having a clue about what you want.
You will figure out what you want when the time is right, just like you will meet that special person when the time is right. until then, maybe just do what makes you smile for now.
