I find it peculiar that the Trump-loving grandparents had no qualms about guilt-tripping their son into cutting financial support, thereby indirectly blackmailing their grandkids into attending the family gathering.

They don’t seem to care what their grandkids want; instead, it’s all about how they feel about the situation, even if it’s at the expense of their own

flesh and blood.

Seems like they are pretty typical Trump voters, by the sounds of it. You've answered your own 'peculiar' question - they can do it because that's how they think. It's the cult of "I'm right and everyone else is wrong".