She is lucky to have a big sister who cares about her like you do. I hope that she gets to stay with you. You may not have that much choice if the authorities do decide to return her to her birth family, but that doesn't sound like a family that it would be a good idea to reunite her with. It sounds like she is better off with you. No matter what the future brings, I hope that you two will always be sisters and always love each other.