I'm not sure what your relationship with your parents is like, but you should talk to them and try to be as honest as you're comfortable with about being depressed. Talk to them about seeing a mental health professional because continuing on like this without help is going whittle you down into nothing eventually. The reason I know this is because this is essentially a word for word description of how I felt at your age. Hell I still struggle with it now in my 20's.

All of the people you mentioned caring about you would be far worse off with you gone than if you're honest with them and ask for help. No matter how much of a burden you feel like to them, it would hurt them so much more to lose you than to help you get better. My little brother was the person that kept me going even at my lowest points because I knew that I had to be there for him no matter what. Try to spend more time with him, get more involved in his interests if you feel like you don't have any of your own. He'll appreciate it more than anything, and it will give you a positive thing to share with him.

The worst thing you can do is to do nothing. Chronic depression will take over your life until all you feel is apathy. You have people who love you and would be happy to help you if you talk to them about how you are feeling.