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?

Agree with the above answers. I wont worry about her treating the children differently, because she went through labour for both of them herself. Both are her flesh and blood. BUT are you able to take her elder child as your own child, and not show favourtism? If you end up showing favourtism, u might end up forcing your wife to side with the other child more. So you have to love and treat both child equally.