Supporting niece through abusive relationship
First, and most basic: To know what your sister would do is impossible, so you can only do what you think is best. Give yourself that permission.
Second, your niece's poor judgment is costing her dearly without your having to attach a single consequence of your own. She has been friend-dumped, family-estranged, publicly humiliated, strung along. If she were ready to receive a punitive "this is a really bad guy" message, then she would notice her mailbox has been overflowing with them for years.
Third, your niece is 43. With a younger adult, the anguished parent figure can justify more forceful meddling, if only on a onetime basis. Those within a decade of AARP membership, though, for better or worse, have standing either to be confronted as peers or let be.
So there's your choice: peer intervention, or punt?
I recommend the peer intervention recommended to me most by survivors of abusive relationships: Tell your niece simply that you love her and that, when she's ready, she can call you and you'll be there. Day or night.
It conveys two essential things: Yes, this is an emergency, and yes, you're safe with me.
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