Searcher60 I think you make some interesting observations but I'm afraid i think that you are wrong in your conclusions. Younger people may not choose to socialise with people who "don't fit in" with "their" group but surely WE didn't choose this either - and still don't. We may be polite and friendly but I'm pretty sure we wouldn't choose it.
As for being insular, in my experience they aren't. They seem to have more diverse friends and relationships than most of my generation and mix more across "social classes".
Also, I feel this is not really what the post was about.
The OP was feeling a bit left out, not put-upon. And I don't think this is an ageist situation either, more about them having time as siblings together. They do include Alexa in things and she says that they are caring and attentive. I'm genuinely sorry if you have had bad experiences with the next generation but I don't think it's universal by any means. Take heart!
Alexa, I expect it's hard to be on your own when they are having fun, but as others have said, it's great that they get along. I hope you get to enjoy an outing with them very soon!