She probably thought you would have ran screeming if you had known the truth, and then she is stuck in the lie. I would bet this eats at her all the time, but she doesn't know how you will react if she tells you the truth. I would suggest letting her know that you know in a loving way, "honey I know you lied to me about how many times you were married, but It doesn't change how much I love you, did you think I wouldn't love you if I knew?" this gives her a non combative opportunity to explaine.
Good luck







