It sounds to me as though she wanted to be with you guys, but not as a "married triad". It also sounds like she may have felt she was intruding on you and your husbands relationship. Perhaps she wasn't ready to be an "equal". She seems to have wanted to keep her life separate. She obviously has an outside influence or at least an outside pressure to still be or pretend to be someone she is not (mother).
I'm not sure if my input can be of any help, since it might seem like I'm just repeating what you said, but sometimes a change of syntax can put a new perspective on a problem. Time and communication are about the only thing I can suggest. Ask her what she wants for the relationship, what her ideal would be, what part she sees herslf in.
Life is about the journey and not the destination,
so what better way to know life
than to wander all the roads and paths set before you.