Hi again -
I would contact an attorney on this issue but here is my opinion on where I believe you stand:
Being that your father is still alive it is doubtful that she has a case. There may be a letter but it states that it was his intention for the house to be conveyed upon to you upon his death. After he dies is the key point.
Did your father ever legally deed the house to you? Who’s name is on the deed? Did the letter specify that the house would go to you or that the house would go to you and your wife? If this was going to be a gift or an inheritence from your father and he had followed through on doing so legally it will make a difference if he gave it to solely you or you and your wife. Look at the home page of www.rosen.com and go to the divorce library and look up the different types of property in the section on splitting assets.
Also if he legally gave it to you have you ever invested marital funds into the house, i.e. a second mortgage?
Where you and your wife paying rent or a mortgage or did you live for free? When you pay rent or live for free you do not build equity in a property.
Best of luck again