- People generally cannot be argued out of positions using facts/logic/science, if they didn't use facts/logic/science to come to that belief in the first place
- As cold/brutal as it may seem, do not set yourself on fire to keep others warm: if the rest of your extended family is beyond saving and won't listen to reason, you must prioritize yourself, your partner, your children, etc.
Now, with that being said, I think it's worth having a final "come to Jesus" type of talk, and adopting a very frank/blunt attitude. I would try something like this: "Mom, I'm not going to argue with you about whether COVID is real or not, how serious the risk is or not. What I want to do, is discuss how to handle your affairs, should you go into the ICU or die. Is your will in order? Which hospital do you want me to take you to (knowing that some are filling/full)? Do you have a burial plot picked out?". Make a last-ditch effort to shock them with the realities of how seriously you are taking things, how matter-of-factly you are preparing to deal with their likely illness or death. Don't make it a political debate, and just tell them, if you do things your way, I want to know how you want me to deal with the aftermath.
It probably won't work, but I would say do your best to make them confront the likely consequences of their choices. And you could also try to show them more articles like these [1], [2], or [3], though I'm sure you've probably already tried.
[1] https://news.yahoo.com/im-doctor-tried-plan-extremely-204117...
[2] https://www.statnews.com/2020/12/31/judging-patients-covid-1...
[3] https://www.nbcnews.com/science/science-news/how-talk-your-f...