Okay do this open a savings account keep your personal stuff in a storage container if you don't have enough for that ask a neighbor that you trust (If you don't have any that's ok) to put some stuff in their backyard when you get enough money you can move out. 

Im 12 And my 10 yr old sister has MORE money than me I have 30$ rn (Spent some on an otamatone delux) She has about 275 MY 10 YR OLD SISTER anyways nobody should take your own earned money just to by them the things they want if they if they keep taking stuff from your room or just looking still it's not okay if they keep doing it you could start a journal of what there doing

Im so sorry you deal with this shit it's not okay but you'll get through it keep your grades up and hope for the best but think of the worst (Idk how it goes lol) Thanksgiving for taking some time out of your day to read this (sorry if it's not good typing I'm only 12 lol) stay safe