I don't know how to describe the sort of person I'm getting the impression sister is. I've only run into a few of them and never anyone I've been close to because I find this trait off-putting. They are people who regularly *hide* things that are trivial and have no need to be hidden. I'm not talking about someone who genuinely doesn't think to or forgets to mention. It's a person who acts as if it's very deliberate on their part not to say something. After awhile it just gets creepy and weird.
This is just a guess on my part that the sister is this way from the LW's description.
I'm trying to figure out why this is something where it matters one way or another whether it's "hidden" or not. It's so trivial as to be ridiculous. So what if the sister "hides" this fact? Why is it an important fact in the first place, one that must
Hiding a substantive piece of information is one thing -- I could see being upset if the sister had seen the son robbing a bank -- but going out to a restaurant? And if there's a pattern of "hiding" here, this
is the best/most egregious example that the LW could find to submit? It makes me wonder how trivial the other examples are. "My sister knew my husband went to the bathroom and didn't tell me! Waaaaaaaaaaaaaahhhhhhhhhh!"
Like I said it's hard to describe. It's like if someone asks why they didn't say, they act smug, like they are playing the childish "I know something you don't" game. Often they just shrug and/or offer no explanation. If they explain, say they forgot or didn't think it was worth mentioning or even say what you've said, it comes off as a lie. If you saw it you might wonder if they had been up to something themselves and were hoping it wouldn't come up, but it's more.
So? Behavior like that is so horrible it warrants a letter to an agony column. Besides, I think you (and the LW) are projecting your own feelings onto the other person. Are they really "smug" or are you simply interpreting it that way. Calling an omission like that a "lie" is way over the top. I'm not a video camera, set to replay every event in my life or else be accused of lying.
I get not liking secrets -- I don't like them myself. But this doesn't even begin to rise to the level of a secret.
A lot of people I know just chat about random social encounters. I could easily see someone saying "Oh I saw your son the other day, but he looked busy with some associates so I didn't say hi. I didn't want to bother him." Most of us wouldn't grill each other about that. I'm not talking about people who have external reasons, like people who would grill them.
Again, why is it so important that every
encounter like this be told? That's what you and the LW are implying -- that every
encounter must be told to you, or the other person is misbehaving.