Best friend that is so close to you, she is like another wife your husband married. Husband may mistake her for his wife when she and wife are in the front seat of the car and said husband is in the back seat. Inflection and speech begin to mirror each other. Your kids may favor her over their own parents, and she treats them like they are her own! Not to be confused with sister from another mister.
My husband does not know who he is going to see first when he gets home, myself or my sister wife!