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Hi- sorry your kitty is losing some hair.
I recommend addressing this. Is she on flea control? Any new pets in the house?
Any people in the house with skin issues?
Good that the people are okay. Hair loss in cats is often from overgrooming. So usually they are itchy from something. Even indoor cats can get fleas, and you may never even see one flea. So my first thought is to treat her and every pet in the house with Advantage or Frontline. Good that she is on the same food - though cats can develop allergies to the same diet they have been on. I don't recommend a food change at this point, though - that is less likely.
It is possible she could have a fungal infection - again less likely, but not impossible even for an indoor cat. Something to consider if she doesn't get better.
Cats can also develop allergies to their environment. Treating this is not so easy sometimes - antihistamines are not always effective in cats and oftern we end up using steroids. If her condition is mild, I would not jump to that.
If she has no scabs, no redness, no spreading of the hair loss, you could try flea control and monitoring. If it's getting worse, not improving, spreading, people in the house are getting skin lesions, then she needs to see the vet. You could certainly take her now, too.
My plug for eleven year old cats - in general, they should be examined every 6-12 months, and have bloodwork annually or semi-annually as well. If she is due for an exam, then now might be a good time.