The most common problem in my area leading to hairloss on the top of the cat is flea allergy dermatitis or other allergic skin disease. This would not be contagious and there would be no risk to the family. Determining if it is going on might allow her to return to being more a part of the family. I don't know of any sure way to help you decide which skin problem is more likely at home. It would be best to schedule a visit with your vet as soon as it is feasible.
Skin disease is usually a lot easier to treat if treatment can be started early in the disease.
Inc. Opinions expressed are those of Dr. Richards.