I am in a serious cross roads here please help me?

I am really not sure what to do here. I will explain my situation without being long winded as best as possible.

I would say in the past 5 years I have come to lose everything expect my little sister and brother (she just turned 10 and my brother is 15) but because of drugs and moving from home to home, I will not go into details, but my father has not been around for those past years and my mother is basically not in control of herself and will be leaving for rehab.

My grandma has decided after talking to our old social worker to take care of my sister and brother from my understanding though we have not moved, I currently help take care of my siblings and help bills/rent. I figured after they move I will go on my own and take care of my own business.

This is where I have problem I am not sure 100% if they will be ok, I have in the past year or two become the one to say yes and no… I bought them Christmas,try to help them out with stuff, always breaking up there petty fights, and bug them about there grades in school. My sister is doing good but my brother is sipping in math a bit. I worry that my grandma means well but truth but told we hardly know her and I am not sure if she is going to but taking best care of them. and my sister is getting more older and I worry alot about what seems she is already having some body changes, if I do not talk to her who will?Whos looking out for them?

My brother and I are on good terms with this subject.

Like my brother needed a field trip for class, and If I had not given him the money would? My sister wanted some animal books (shes loves animals) who would?

Alas I feel very guilty of leaving them behind, as I wanted to go to school for music ( I know dumb choice, theres no money there) but I also want to follow my heart. but then I start to think if something happens I will not have the money to support them then what do I do?

I have considered military but I will most likely be shipped around a lot and would not be able to be there anyway and that not fair to them. I am not sure if its for me.

I am 19 right now if that makes any difference, least with being adult I can gain custody if need, yes?

What would you do in my situation?

If not for my brother and sister, I would not even ask this question I’d go after what I want, with no worries but I feel like I have become a fill-in parent or something….so I have to right?

I really don’t know what to do, I have no one to talk to about this….I need to figure something out…..