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 children.. there won't be much difference. But in fact , is how are u going to treat her 2yo baby.. like u will still treat it as ur own kid. u have to have more hands on in handling the kids.. the way u interacts with her 2y baby.. I personally as a wife, I would observe n judge.. if there is any mistreatment frm my current hub to my elder son whom is frm my previous marriage.