Adoptions within families are not uncommon. If you choose to do this, and she agrees, she must also agree to, and sign away, her parental rights to the child. After that it is up to you and her and your family as to how to tell the child in the future. It's best to, because it will come out.
well i think you got to go down to the courts and if your wanting to legaly adopt him i think the mother and father have to sign off the rights and i understand i would want to do the same i mean thats ur nephew my brother and his old lady were going to get rid of there baby and i told them no thats my nephew i will take him i mean thats my blood but they changed there minds and have been wonderful parents so you just got to think what ur sister is going to feel and want to do later on down the road as that baby gets older and ur sister becomes more mature