Antiwork

I recently joined a company and when I joined there's another girl in the similar position and we both report to X. After few month I found out the girl and X were is some sort of weird sexual relationship whereby they will stay back after 5pm. Go for lunch together , girl all dolled up on X's bday. Now X moved to a different department because of a project and made the girl ,the manager. Now girl is dumb af. Doesn't understand anything technical nor have any managerial experience. The only reason she's a manager is because of the relationship wirh X. I am now frustrated and overworked because of her poor decisions and no managerial skills. On top of that when I joined she has more than 400 task pending and because I have been closing all my task , she asked me to close hers. I did some but because of her erractic behavior and because she creates__ a toxic environment. I stopped and now she's asking me to work on stuff which I haven't got any training on and no support from her. Really frustrated and just want to quit.