My sister lives with my husband and I, we live in the basement apartment of my mother and step-father's house. My mother is currently out of the picture. My sister's father (not mine) is now threatening to take her, out of the county we live in, away from the school system she has gone to for 8 years. He has custody of her, what can I do?

You will have to file for an order of custody of the child in your local jurisdiction. It would probably be a good idea to hire an attorney. Document everything and let a judge decide what is better for the child.

You sister lives with you and your husband in a basement of your mothers home ( but you mother is not in the picture?? )

Your step father has no rights what so ever.

How old are you ?

Where is the mother ?

Most likely you as a sister can't do anything. If the father has legal custody and wants the child

If there is nothing saying that he can't have the child then its up to your sister to file otherwise he could come and take the child and if you were to withhold the child from him you could be charged with kidnapping and face going to prison. You have no legal standing in this matter at this time.