My DM has fallen into a similar rabbit hole. She will expound nonsense as irrefutable fact, and if I ask her for the source, wil say 'the internet' and look at me as if I am wilfully thick. She lost her (standard organised) religious belief some tears ago, and was flailing round with mysticism and wellness so I think she was primed for some kind of organising influence. Covid was damaging, as well as lack of social input - too much time on the internet.
She isn't stupid, she isn't male, she is not depressed and she was never right-wing although I think she has become a bit MAHA-adjacent. Sometimes she says thinks that I think are emminently sensible. I think that one size doesn't fit all. I wonder how much of this reaching for madness is a way to control anxiety about the world, lack of her control, her place in it...
My dad (a scientist with an analytic mind) puts up with her - he is still deeply in love and emotionally and practically dependent on her. I and DSib roll our eyes, try gentle challenge when we can be bothered and make it clear that we don't agree. But it does affect my relationship with her- I am less keen to see her, especially in settings where alcohol might be involved since this disinhibits too much. I do occasionally say 'i'm just not going to talk about this' and physically leave. I do wonder where the ultimate limits of my tolerance are. I don't have to live with her so this is easier, and most of the time I am able to pretend it isn't an issue.
I try and encourage her social life as I do think the Real World is the best antidote. In a way I wish it was a 'normal' religious belief which would provide structure and community and some boundaries, which this freewheeling crazy collection of 'facts'.
All this to say, I'm sorry OP. It must be really hard to live with. If you think that anxiety/a sense of personal unimportance/mortality/lack of control/loneliness may be underpinning this, can you help with these? Foster real world connections and esteem? Can you put in boundaries to protect yourself 'I hear what you are saying, I don't agree, unless you have overwhelming evidence I will never agree, can we agree not to talk about it?' Just as the drive to conspiracies is not simple and universal, the solutions probably aren't either. My great sympathies.