You are wonderful for raising her.

My dad died when I was 16. Thankfully, I still had my mom. My mom dies when my brother was 10, but he still has his dad.

The death of a parent at those ages is so incredibly painful. I think this child is going to push as far as she can and it is going to be VERY important to maintain boundaries and expectations.

I agree that family therapy is probably quite necessary for you guys to succeed.

Don't try to be a replacement for her mom, and make sure she knows you don't want to replace her mom. Even go so far as to say "I will never try, nor do I want to replace your mom. I gave her my word that you would always be taken care of and loved, so that is what I am doing" This is something I have to frequntly tell my brother, along with "I know you don't like to have to follow the rules, but it is my job to keep you safe"