I'm not missing that point at all! I understand exactly who is blood related to who! The grandparents are related to the second set of grandkids, but not by blood. They are related because their blood grandchildren are blood related to the second set of grandchildren, by both blood and lived experience. Putting "respectfully" in all caps is not respectful.

"my point was that they should have been acting like the kids were family for years already, as the kids did nothing to be treated as irrelevant by the grandparents. It's not their fault that they exist."

I'll repeat myself again, so you can see that I know the relationship, but that I think it's rude for the grandparents to be treating children like they're not family at all (which they are BECAUSE they're the siblings of their grandchildren), and barely interacting with them.