Consider if it’s worth having these conversations in the first place. Realize the best you can accomplish is to razz your conservative relatives a bit. If you think they can take it without it becoming a significant issue, the communist manifesto has brought more people around to socialism than anything else so start there. Don’t cite anything specific as that’ll just cause them to shutoff completely in most cases, play the rhetorical game foremost. If they act like they’re already super familiar with Marxism, ask them about their thoughts on historical materialism or maybe the tendency of the rate of profit to fall in capitalist economies—asking about specific core Marxist concepts is rhetorically useful since it’s impossible to learn about Marx and not have encountered these phrases, making it a good way to reveal their actual knowledge. Don’t take the debates too seriously and if people start to lose their cool consider just letting it go.