I need advice asap. life is hard, death must be easy....?

i am 17 and ill be turning 18 next month. ive had a job since i was 16 so i started taking care of myself and moved in with my sister. she is 20 with two kids and our parents are deadbeats. we are pretty much on our own and struggling financially. i quit school to get a job and help her financially. i will not graduate with all my friends and i feel left behind. but ive put my pride aside to help out and take care of my responsibilities. someone recently offered me a deal on a digital piano 200 for it when normally they are very expensive. playing piano is the only thing that makes me happy and its my love and pion. i had to give it up when i quit school though. my sister says we cannot afford it because we are trying to move and we have a lot of bills right now. i am crying because i really want it and wish i could do this for myself but i know i cant. i wish i had parents who could help me.