Pixel might not ever be accepting of the kittens. Cats are very territorial. Even though she had crystals in her urine, I'm sure the stress made it worse.
I agree with Candace: Feliway might help, it also comes in a diffuser you plug into an outlet.
Also calming treats like Composure are an option.
If you are not giving up fostering (which is a wonderful thing to do, but might be the only way to make Pixel happy again), make sure to watch out for other signs of stress in Pixel, like excessive self-grooming and changes in appetite.
You are already giving her as much attention as possible, keep that up, even if the kittens get upset.
Good luck to both of you. Pixel, I know how pesky kittens can be.