My sister can no longer provide for my nephew who is 16 years old. I told her I would take him in and provide for him. How do I go about becoming his Legal Guardian or Custody of him just so I can make the decision for him as well as proof of responsibility of a minor for thie military?

Go see an attorney to draw up the Guardian papers for you. Very simple procedure to do.

What state? You can look it up the procedure for guardianship on your state.gov or we can.