Tom, in a situation such as you describe, there are a few options. The obvious one--that she is in a controlling relationship and that her husband is choosing to isolate her--has already been pointed out. Another possibility is that she was abused (or believes that she was abused) by someone in your family and blames the family for not protecting her. A third alternative is that she has experienced a change in her mental health and she may be acting on feelings that are delusional.

You don't have a lot of good choices, given that she has largely severed contact with the family (which, as a presumably competent adult she is at liberty to do). Do you have any mutual friends who might be willing to speak with you in confidence if you approached them--and if you assured them that your only concern is for her welfare?