Re: How do you deal with ....
With what you've written about your mother, I'm sure you can see why your father was attracted to her in the first place, given that he is a narcissist.
My mother was a narcissist. Plus she was an alcoholic and both emotionally and verbally abusive. My father would never stand up to her. When my siblings and I tried to defend ourselves against her verbal abuse, my father would take us aside and tell us to stop. His reasoning for this was that if we argued with her, she would yell at him later for hours on end and he wanted some peace in the house! Yup---peace in the house was found by throwing your kids under the bus.
It's been 11 years since my mother died. My father got remarried a year after she did. He picked another harridan and ended up divorcing her about 8 years later. Sometimes he talks about our childhood and tells me "Your mother treated you kids very well." When I disputed that, he said "She was only like that sometimes."
So...like my father, I'm sure your mother doesn't really get it. Although I give your mother points for leaving him, it's sad that she didn't get any counseling to deal with her issues. In the end, it is the kids who suffer the most.
Perhaps it's time to sever contact with her and your uncle. It's not easy to do to be sure, but you have your sanity to consider. If you're unwilling to totally sever contact, at least keep it to a very bare minimum.
My mother used to tell others lies about me. According to my father, he claimed that he talked to those who were told the lies and corrected the lies. Since I never witnessed him doing this, I can't be sure.
You've tried to tell your mother that your uncle is a liar but she has chosen to take his side. Sadly, there really isn't anything you can do about that. Perhaps she will have an epiphany and realize what a liar he is but I wouldn't count on that.