My wife already has a two year old baby and is expecting our first child soon. how do I make sure that she is fair to my child?

Pardon me if i may sound rude. Both are her kids. What makes you think that she wont be fair to your child. It should be the other way around actually. She should be worrying if you would be fair to her other child. Nevertheless blended families are tough. I know it, as im in it as well. Blessings!