The thing about rationalists is that they are fully invested in irrational beliefs, which they prefer not to examine. In other words, just like most people, but with a specific terminology that, if they use it properly, identifies them as one of the elect.
I suggest that whenever your relative talks about EA, you talk about kindness. When they bring up longtermism, point out that you have to survive in the short term to reach the long term, so working on better policies now is rather important. If they start in on life extension, note that until quite recently, all the major advances in improving average human lifespan come from improving infant mortality, and be prepared to explain the demographic transition.
When they go extropian, say that it's a nice vision of the future but your kids are unlikely to see it unless we fix the world we're currently in.
But most of all, point out that multiplying infinitesimals by infinities to justify any course of action (a) is Pascal's Wager and (b) justifies every course of action -- so it can't justify any.