And what are you supposed to do when several of your relatives do not exercise this level of restraint and insist on steering the conversation toward politics?

Turn the other cheek. Steer the conversation back towards something less contentious. If necessary, confront them... but not as a partisan. Challenge them to abstain from political discussion in favor of a happier conversation. If they really cannot be adult enough to make that choice, then it really isn't politics that is the problem, it's just a symptom.

At a certain point you're just tired of that and want to excuse yourself and go home.

why, explain vehemently just how naturally sane and correct your opposing opinion is, naturally. if they wanted the right to speak, they’d have the right politics in the first place and the really important thing is to make sure they know that

Are we talking about having a nice dinner with someone or are we talking about petulantly asserting our "rights"? Someone is well within their "rights" to not stay somewhere they are not being made welcome.