I wonder, is it rude to tell her that you step kids have formed their own traditions with their nuclear families, so are unable to attend her festivities?
Is there ever a polite way to say, "Hey, you rejected us, so we moved on and are doing celebrations with our own families, so now you're reaping what you've sown?" That is something I've often wondered about.
I think it would be acceptable within etiquette to say "You didn't want us there/didn't approve of our religion, so we moved on and are celebrating with our own families, and we're not going to break that up now." The stepmother might not like being reminded of it, but I don't think etiquette means never mentioning that there's an elephant in the living room. The explicit "you're reaping what you've sown" might be over the line; a calm "you rejected us because we didn't share your religion. Maybe someone at your church has room for Thanksgiving guests" wouldn't be.
Stepmother may now have come to realize/agree that family and hospitality shouldn't only be on the basis of DNA, and if she explicitly said "I was wrong, family can and should be larger than genetic relationships
" the OP or one of their siblings might want to make room for their stepmother. But Stepmother doesn't get to just invoke whichever set of rules works for her: what "family is by blood" people don't always realize about those who believe in some version of family by choice (whether it's "you're still my sister-in-law even though my brother passed away" or "my cousin's father is still my uncle, even though he's not married to my aunt" or "this child is my nephew because he, I, and his parents all want him to be") is that between adults, that choice is mutual. If an adult tells someone "you're not my family" it's likely to be believed, and that doesn't come with the option of saying "I didn't say 'cross my heart and hope to die'" years later.