Ideas
I also have this issue with my current save, which is a legacy. I'm playing with no mods or cc.
I recently moved out my next gen sim to live on her own. She's part of a family club with her parents and some extended family, but there's no room for her siblings, so I wanted to create a club just for her parents and siblings, but when I go to add sims, the only family member I can add is her mother, no one else, not even other extended family show up as options to add to the club. The only family member shows up in the list of friends is her mother, her siblings and father don't show there, and if I choose the family category again only her mother shows up, no one else, not even the extended family. Before it's asked, they have great relationships with full green bars and is best friends with a couple of them, so it's not a relationship issue.
This isn't the first time this has happened, when I was creating the original family club I had trouble with this as well.
There are a few reddit threads talking about this issue, so it's not new and it's not unknown. One workaround I found online said you might be able to invite someone to the club if you're talking to them, which, ok that's a workaround, but really that's ridiculous. How does my sims close family, who she knows, not show up to add, but sims who she barely knows are available to add. It's very frustrating.