The very first question is not what the "aunt" thinks about adoption but what the Mom and Dad think about adoption. Unless the Mother surrendered her rights I doubt seriously that she has had her rights terminated in 16 months. The father may be in prison but that doesn't terminate his rights nor is it, in and of itself a reason to terminate his rights. So I really think your sister is getting the cart before the horse.
The Aunt's opinion is nice but does not make the child available for adoption. You can not force this issue. Both the Father and the Mother have very strong rights here, and simply because your sister wants the child and thinks the child will have a better life with her is not grounds for adoption.
So get things set up first. Get the aunt to put you in touch with the social worker.
1. What is the status of the Mother and Father?
2. If their rights have not been terminated, would they consent to adoption?
3. If both of those answers is no then there will be no adoption till one of those things happens.
4. Next you have the hurdle of family members who might want the child. Obviously the aunt doesn't but someone might and they have higher standing than your sister.
5. I would warn your sister not to "bond" with this child just yet. Emotional bonding with a child that isn't yours is unhealthy for her and the child.
6. If the parents have surrendered their rights or are willing to, or if their rights are on track to be terminated, then approach the social worker about adoption and she/he will put you in touch with the adoption unit. There will be classes and home evaluations and you still might not be able to adopt that child: there may be a line of people waiting to adopt.
Just tone down the "bonding" thing until you know the child is potentially available for adopting. The aunt has NO say so in that.