I'm currently married with one high school daughter. My sister is in a different state with her own daughter. However, the child is currently illegal. We were talking and my sister is willing to let us adopt her niece so that we can arrange citizenship papers for her before she graduates, or else she will have a hard time. Would this be possible, and if so, how should we go about doing the legal work?

This seems like a very difficult situation. It is not possible to adopt someone who is not in the country legally. Domestic adoption requires that the child was born in the United States. If not, the adoption is an international adoption and is subject to the rules of the Hague Convention on Intercountry Adoption. There are strict rules regarding transportation of children between countries. All children must be documented and in their country of birth before an international adoption can be approved.