First off welcome!
Just wanted to say that what your describing is typical kitten behaviour. She is very very young to be off her mom and without her siblings. They get such value from this in the later weeks after birth because they get experience from being with their littermates and nutrition from mom. Most kittens dont come off the mom ideally until 7 -8 weeks but I know many go sooner than that.
The other concern is I dont think she should be vaccinated so young? I could be wrong but it seems to me you shouldnt. I am positive some of our experienced kitten folks will give you some more advice on that.
Have you tried redirecting her biting to a toy? I think you may have some trouble when your older cat gets to know the younger one? some dont mind it but others have more trouble with it. Kittens are so full of energy too! you may find yourself giving the kitty a time out somewhere so the other one can have some freedom... 
Most importantly thank you for taking her in. It is so sad how disposable animals have come. Your her little kitty angel now! I think as she ages you will see a signifigant change as she matures.
Good Luck
Cindy
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"UNTIL ONE HAS LOVED AN ANIMAL, PART OF THEIR SOUL REMAINS UNAWAKENED"
He is your friend, your partner, your defender, your dog. You are his life, his love, his leader. He will be yours, faithful and true, to the last beat of his heart. You owe it to him to be worthy of such devotion.
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