I am so happy that you have your 'big sister'

I don't believe that at 15 you should be the "mother, sister, cook, cleaner or care giver" which is the situation you are in.
Sure its nicce to help out.. do the occassionaly load of washing.. tidy up the house etc but I have to say that most people your age are actually having fun.. and you are not. Sure I did chores at your age but i got something in return be it pocket money, a hug or just some recognition of what I had done. You deserve to be living a life of your on chosing, hanging out with friends and having a fun time. Believe me the teenage years (along with all the angst and hormonal changes) don't last forever and I look back at mine with glee! I was a terrible teenager.. and did all the things you are not meant to do but geeze thats what it is all about.
You are grown beyond your years.

Take care of yourself and please find some time for YOU.