Talk to the child's case worker.
If you're in Arkansas, you and your sister can request an in-family placement. If you're not in Arkansas things become more complex, although depending upon the full facts and history of the case it may be that the child can be placed with you, for example if the court is amenable to dismiss the petition based upon a voluntary placement.
If the case worker is resistant, if the present out-of-home placement has been in place for months or years, if you think a court might rule against you, or if you're out of state, you should consult a lawyer.

