Hypothetically, would we get child support?

This is just something my husband and I were talking about, and we weren’t sure the logistics of it.
Let’s say something happened to my sister in law. She has a three year old. My husband and I would want custody of her.
Now, the girls father is in and out of jail, cannot hold a job, has never paid child support…so we’d have a great case.
So, if we got custody, would we adopt her as our own? If we did that, would we be able to get child support from her biological father? Or would she be considered out daughter once we adopt her, so we get no support?
I’m just curious!