well you should be able to but if not then just do it secretly until you can but if you don't want to do that then have you mom adopt her under your name
NO you can not adopt ONE, you can adopt children and pets though!
The parents have to give up their parental rights and you file to adopt the child in the county court house where you reside.
The process varies from place to place. You will need to contact the local authorities.
Since the individual is over the age of majority if they consent to be "adopted" it would be lawful.