My Wife Betrayed Me.

Dear J.A Blog. I am a married man. I love my wife and vice versa. Our marriage is five years old and are blessed with three children. Ever since we got married, my wife had brought a girl into our house, which my wife claimed was her sister's daughter who died some years back. We did take care of her like our children. Recently, I heard from a reliable source from my wife's family that the girl was actually my wife's daughter, which she had to someone else before I met her. I view this as unfaithfulness on the part of my wife. What can I do? Please advise.