It may be allergies to food or fleas, or other allergy. You don't have to see the flea that affected your cat since one flea bite can causing itching for at least a month if the cat flea allergy dermatitis; and the flea could be brought in on clothes, etc.
There are some things that you can try at home, but, if they are not effective I encourage you to take her to the vet no matter how far away.
I would switch her food to a hypo-allergenic, grain-free, novel protein, CANNED diet. You probably need to switch both cats to avoid possible cheating by the affected cat. If she is indeed food allergic it will take up to 12 weeks to get the allergic food completely out of her system, so be patient.
You will also need to eliminate fleas just in case with a topical flea preventative and a premise spray and bomb.
But to reiterate, if these measures don't help, please see a vet as there may be a congential problem, or other issue that needs to be addressed.
Thank you so much! I will message back with results.