I don't know. Are your parents involved? If they are, they will defeat any request you make. If there is some sort of abuse charge thay are facing, you may be able to go to court and request custody.
You cannot obtain "custody". You MAY be able to establish a Guardianship for your sister; with you as her appointed guardian. I question whether that would be cost effective given that she will be a legal adult in 2 years.
If you are caring for your sister you could seek a temporary guardianship if your parents are agreeable. Since you don't say why she is with you, it is difficult to provide a further answer.
If the parents agree. If not, you will have to file a petition for Custody and prove to the. Piet why it is in the child's best interest for you to be awarded custody.
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