Im a teenager "wishing my life away"?

You are not mental, but you are putting the cart before the horse. Look at what you have written, your sister is 25, you are 15. You have time for all the things that she is experiencing. Babysitting is a lot different than taking care of a baby fulltime. Having kids is the best and hardest thing that you will ever do, Now is not the time for you. If that is your goal you need to get yourself prepared for that in the future, first and foremost right now is completing your education and securing a job for yourself that will allow you to support yourself and a family. Besides, take the next few years to have fun, once you have kids they are the priority, your fun and your needs get put on the back burner. Be young and have fun. You will change so much in the next few years, enjoy your young adulthood before embarking on parenthood. You won't regret it.