My brother is in prison for child abuse with major bodily harm and my neices mom did it she almost died and?

Everything. Sad thing is that she was only 3 months when it happened. And now she is a little more then a year old with alot wrong she's missing a quarter of her brain almost died and the back of her skull is really soft. My brother is innocent but loved that ***** so much that he took the blame for her and is serving 15 years but actually 23 with parole and all that. Anyways she recently wrote him a letter saying that shes sorry he's in there for something he didn't do and something that she did and she was so sorry and wishes she can tale that day back. My question is can he file for an appeal and show that as evidence that he is innocent?