I understand she has been through a lot, but I'm also hurt deeply by her treatment. If this were a friend, I'd sadly assume the relationship was over, but this is family and I'm under a lot of pressure to make things right by my wedding. I'm at a loss as to how to prevent this relationship from deteriorating further and conflicted by my hurt feelings vs. her grief. Am I being unfair? Do I ignore her behavior because of the understandable cause of it?
Answer: It's good she's not a friend, because I believe your friendship-ending assumption would be premature.
"Tara" (I'm naming her, for simplicity's sake) suffered a "tragic loss," and she has written off at least one other person besides you — and the fact that you noted these means you recognize, on some level, that this is more about her grief than it is about you. If anything, it sounds as if you want a push one way or the other — in the form of permission to hold this against her, or encouragement to redouble your patience.
I also suspect you want this push because the wedding looms, the family presses, and your patience is ticking down like shopping days till Christmas.
However: Like Christmas, the wedding is an artificial deadline — a manufactured climax to the story of you, Tara, and her grief, a story that's bigger than any one day. Ten years from now it won't matter whether she stood at the altar with you; it will matter how you handled yourself and your relationship in light of her grief.
Accordingly, tell anyone who pressures you to "make things right by my wedding" that there's no deadline here; you love Tara and care about her and will give her all the time she needs.
Tell this to Tara, too. Explain that whether she stands with you or not is beside the point (though you'd love her to) — the point being that she gets the care and support she needs from you. If she can articulate what that is, then great, but if she can't, then you'll need to listen to her in more subtle and sensitive ways.
In other words, the way to keep this relationship from deteriorating further is to keep firmly in mind what this isn't about at all (a wedding, a family unaccustomed to loose ends); what it's somewhat about (you); and what it is primarily about: "a close family member" who needs all the compassion she can get.
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