I am genuinely baffled about how people get to the point of asking a total stranger for advice before first trying the step, "Ask them to bring something."

In some families it is polite to bring food to parties. In other families it would be the height of rudeness to treat a holiday dinner as a potluck--it would be an insult to the host's ability to plan a balanced and harmonious meal to add an unexpected dish. People from both sides of that divide can easily learn to work with the other side's expectations! ...but only if they're made aware of what those expectations are.

(That's not even touching on the gendered issues of "Why are you focusing on your sister-in-law as the rude one instead of on your brother," of course, which Amy rightly highlights.)