Could I get dual guardianship of my 16 year old brother-in-law in the state of Virignia?

My brother-in-law is caught up in a pretty nasty divorce, and his mother is willing to grant joint custody to my husband and I to help pay for the last two years of his high school. We would like to help so that she can get away from the abusive relationship, but still have rights to him herself. We want to be able to sign paperwork when she is otherwise incapable, take him to get his license, take him to the doctor. We have medical through the city as my husband is a CSO, and she does not. She is also not a legal citizen.