Your sister is trying to get some control over the uncontrollable. Some things that are uncontrollable here: grief, unfinished business, and secrets.
Ask her a hard question: Are you ready to forgive? Her husband can't apologize; the friend may not apologize. There is a debt that she's owed that's uncollectable.
Much of this is internal work that has deep roots in her relationship. Pinning hopes on external interactions won't provide significant healing.
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