I had a stepfather like this. Perhaps not quite as bad, but he was like poison, such a horrible negative person. He was also jealous of me, and jealous that my mother and I had such a close bond. My H and I would drive home from seeing them and the whole 2 h drive we would be talking and shouting, telling each other how we'd felt the whole time being with this guy. My H called it detoxifying ourselves.
My mother was part of the problem. She stayed with him and kept telling me how he loved me, and I kept thinking, "Really?!" But I wanted to see my mother, so I let it go for a long while. I did have a few very serious bust-ups with him when he went too far, like major face to face shouting matches. He would get a bit better but not for long, because he was just a miserable shit person. Finally, when I had my first kid and he came to the hospital and just completely poisoned everything as per usual, I hit my limit. I told my mother that I would no longer come to see her. If she wanted to see me and DD, she had to come to me.
A year later, I invited them to my place for DD's first birthday and he was extemely nice and even apologised. After that, it was somewhat better, he was still the same arse but a bit more tolerable.
After my mother died, though, I ditched him pronto. He desperately wanted me to be as devoted to him as I was to my mother. No bloody way. I am SO RELIEVED I never ever have to deal with him again.
Maybe you need to put boundaries with your mother like I did. She's being a bit selfish, and it's OK to make things so that it's not too awful to see her.