It sounds like she might have allergies, hence the sneezing.
The most common cause of this scratching is fleas or an allergy to fleas. Even if you don't see fleas, it only takes one to get the cycle started and it continue long after fleas are gone.
If you suspect allergies are the cause:
Cats don't just have allergies to food. They also have allergies to products, household cleaning solutions and fleas.
The vet can provide meds that will help with hair loss, any secondary fungal infections and will make your cat feel better. Prednisone often is used to control any itch and give the skin time to heal.
Some cats get a great deal of relief from a diet change to a low allergy diet. Your vet can help you pick a product that will be best for your cat.
Some cats respond well to an antihistamine, including benadryl.
If you don't have the cat on frontline or other prescription flea product, you will want to do that as soon as you can.
You should also get the cat started on a supplement that contains omega three fatty acids. Good skin starts on the inside.
I hope this helps.