I'm fortunate enough to not have anyone that close to me become part of the cult. Those who have, I've just stopped reaching out. Though that may not be an option for you in this case.
The best advice I can offer is to try to understand what a cult is, and how to work with that situation. A cult like MAGA is inherently irrational, so trying to win people over with rational arguments doesn't work. Here's one resource for how to talk with people like this.