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?

Both are her child, and you are her current husband. Why would she be unfair to your child? Anyway it is also impossible to be fair to the both of them equally as they are at different stages of childhood and need different kinds of attention. I think you should try to be understanding instead and help her to manage the other baby so that you can all enjoy life together as a family and not be so concerned about being fair to his child or your child. Both are your children now.