I think this letter needs to be rewritten:
Dear Annie, I'm a miserable gossip and my much-older sister got to go to a nice restaurant and I didn't. I know this because my son was there and saw her! Why didn't she tell me she was going out for something that was none of my business? And why doesn't she tell me things that other people have told her in confidence?
Sorry, may be just the way I read it.
The "making up excuses that he was with business associates" thing bothered me, too. Sounds like auntie acted respectfully towards her nephew.
There must be a lot more to the story to make the letter writer so upset. I hope, anyway!