Our adoptive sons birth parents rights were terminated. Could guardianship be granted to his sister that is 21?

If those who hold parental rights are agreeable to the idea, nearly anybody can be appointed guardian of a minor so long as he or she is capable of looking after the child's best interest. Guardianship can be something temporary and informal or something more permanent and formal (with a court order).

Your question is unclear as to why guardianship is needed. Is this question posed in case of your death? Make sure your estate plan identifies your choice of guardian in the event of your death. If you are appointing these people in your will, the court will give it great weight when and if the time comes. It is awesome to plan - it is better to consider you will have a long healthy life and perhaps be there to see your son have a healthy relationship with his biological family. He is fortunate to have you now. If you are wanting to turn custody back over to the child, then you should seek the assistance of an experienced family law attorney to determine if court action is necessary or power of attorney is appropriate. Blessings!

Usually, you can grant guardianship to anyone who is legally appropriate. (For example, you can't grant guardianship of a minor to another minor.) In this case, it might be problematic to grant guardianship to the biological mother if DCFS took the child away because of abuse or neglect. You would really need to disclose that to the court, and it is unlikely the court would grant the request. The twenty-one-year old daughter might be a different question, although I do wonder if the court would see her as a proxy for granting guardianship to the biological mother. Not sure about that one without more facts. Also, depending on the age of the child, he will have to approve of the guardianship, although it sounds like he would be on board. You may want to sit down with an attorney and discuss the situation in full. You should really have all the advice possible before making a decision like this one.