To clarify, I'm an only child so have no experience of sibling relationships. The situation I'm describing involves a childfree couple I know.
Childfree couple, late 50's, one of them has two younger siblings (late 40's) who've chosen to have children. The childfree couple are feeling left out as for a few years, their siblings haven't invited them for Thanksgiving/Christmas (we're in the US), because they're celebrating as a nuclear family, with grandparents, or with other friends.
Is that unreasonable? Do you expect to be invited? As I don't have extended family, I've never had this option anyway. Personally, I think the best solution would be for the couple to offer to host if they want their extended family there. That's what I'd do, or book a luxury break perhaps. 