I am concerned that she has not been eating enough overall. Her milk intake by size should be around 100mL/day. Her previous intake was only half of what she needed and this may have something to do with her small size and also her low blood glucose. It's wise of you to have moved her feeding amounts up to a higher level.
I am also not a huge fan of commercial milk replacers, especially for failing kittens. My preference is a much more rich, nutrient-dense food known as Kitten Glop. Recipe here: http://www.hdw-inc.com/glop.htm
If your vet has not already counseled you on the risk of congenital issues with this kitten being so frail and half the size of her siblings, let me be the one to warn you. Often kittens in her condition will barely hang on for weeks at a time until they eventually fail despite their owner's best efforts. I can understand why your vet has made this recommendation for euthanasia based on the history you have shared. The seizures and ataxia (falling over) may be linked to congenital issues or may be linked to her not receiving enough milk/calories in a day. This is another reason I mention the kitten glop because an easily, and quickly, absorbed sugar source is available with this milk that tends to show a fast responser for hypoglycemic and malnourished kittens.
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