At our wedding, my MIL just announced out of the blue that she was doing a rehearsal wine/cheese/buffet type thing for everyone at the church (everyone - family/friends/etc. had come to just hang out as pretty casual) so we all just headed back there. So my cousins etc. who were not part of the wedding party were there. There wasn't an official invitation - but all extended family came - even the ones who weren't at the church - but were out of town and at the hotel. I don't know if that helps, but I think the 'tradition' of it only being wedding party is not always the case these days.
That was decades ago - so it's probably even looser now.
It just sounds to me anyhow, like the bride and groom are not following your family's traditions to a T - and that's ok. They are doing their own thing. I would just go with it.
The part about the bride not asking your daughter to be a bridesmaid - I didn't ask my cousin to be one. She came from overseas and I didn't ask her. I only wanted my best friends to be in it. I have no idea if she was insulted. She never got married so I was never asked to hers.
I think you just have to realize that not everyone follows traditions - that's an expectation you're putting on someone else, that you don't really have a right to. When your daughter marries, she might not even want to follow tradition. She may want to, may not want to. It's really up to them.
My MIL got very caught up in traditions etc. and is still very hurt that we didn't do everything according to 'plan' - and still brings it up. She is hurt my family didn't do certain things she felt we ought to have. **She didn't communicate this until later - and took everything personally. That was her choice. No one set out to hurt her or anyone else.
I get you are upset - I think you feel embarrassed (?) your daughter wasn't included, or is it hurt (are they very close?). I can't tell.
As for this being confusing, and not knowing if it was a mistake, and not wanting to show up if it really is a small affair, why not just check with your brother or sister and see if they know - but be discreet and just say you're just checking. I think if I was really concerned, that's what I would do if it could be done casually.
ETA - I have heard of younger relatives being included in bridal parties as flower girls, etc.
I am not familiar with a 16 year old going to weekend getaways, dress shopping, etc. with bride and friends in their mid 20's (I take it)? (from previous questions).
You mention your daughter was included in another cousin's wedding, but did she really go on these kinds of excursions?
When I was 16, I was in my sister's wedding (just because there was that many groomsmen and I was required) - but I did not attend any of those festivities. Quite frankly, I would have felt awkward and out of place.
That's just my two cents' worth. I definitely had a bond with my sister (a relationship) but some of these excursions/outings are more fun done with your gal pals/peers than a teenager.
There is a period of time between flower girl and an appropriate adult age, where being asked to be a bridesmaid doesn't always work. That may be the case here. Just a thought.