My dad is very well capable of getting her custody if he wanted and he would support my decision of keeping her if I want. He knows my mother is a very unstable person I just don't want to involve him legally. & I will be stationed in one place for 4 years. Thank-you it has been a difficult decision but its the right one for me.
More importantly, it sounds like it might be the best decision for your sister. If your parents are willing to give you guardianship, that would be the way to go. The fact that you will be Active Military is going to be in your favor. You would have to be in a stable place - no taking custody and then being sent overseas.
If your father can get custody he can then give you guardianship although no one can say 100% without know what State (and you don't need to post that now) so you can get medical care for her, enroll her in school.
I try not to get involved oin the personal issues on the boards. For whatever reason your thread has really touched me - I truly don't know why. Maybe because it's a sister, concerned about a sister? I don't know - and emotion has no part on the law boards. I'm the first person to say so.
Go to basic, get stationed, see if you can get legal guardianship.
I will bet you are your sister's role model. Good for both of you!