I think you WBU.
Interesting that you're describing both of them as 'rude' (both the remainer and the newcomer). What does that mean, exactly? Do you think they're rude because you're too passive, or are you worried about things changing and them teaming up and overpowering you with their rudeness?
How do you deal with rudeness in the kids you're assisting? Are you boundary focused and assertive? I imagine you are. And the skills are the same, surely?
Fight or flight, this, maybe? Maybe I seem basic, here, but... are you catastrophizing here? You're imagining the worst, telling yourself it's going to be ghastly, wondering how you will cope? Then that might just be the anxiety. Don't let the anxiety drive you out of a job, unless you don't really want to be there anyway and you're looking for an excuse?
Running away is a viable option, of course, not trying to judge you with what I'm saying here, but, maybe, you could look at it as a chance to develop some further skills. You can challenge rudeness, you know. It's okay to do that. It's okay to give them your opinion, too: just because your in a subordinate role doesn't mean you're a lesser human being. You also have rights, and so if you find a colleague is a total PITA due to them being overly condescending, you can raise that as a grievance. You're not powerless unless you let yourself be.
However... instead of dreading this new person coming along, you could look forward to it. You could make a big fuss of them when they get there. Love-bomb them. Make them a cake, buy them a 'welcome to the office' mug. That kind of thing. Be bright, and sunny, and let all their bullshit be irrelevant because you're a professional?
Fair enough, if you really don't think you can handle it, but... well... you don't know until you try, do you? Give it a crack, see what happens...