It sure sounds like something genetic. If that's the case, then there likely isn't much more you can do for her. That doesn't mean you have to stop trying. It just means you have some tough decisions to make.Marek's is contagious, not all birds will ever show symptoms though, these birds would be carriers.
If she she had a twitch at just a few weeks old, then it could be a genetic disorder and as she's gotten older it is much worse.
You know your girl better than anyone else, and you know your situation better than anyone else, as well. Only you can decide on whether to continue treatment or not. No matter which way you decide, don't beat yourself up about it. You are a good "chicken mom." This whole thread shows that. You do what you think is best and we will support you.