Did the problem in your younger cat resolve or do both cats have the skin condition now? I'm not sure if you live in the U.S. or another country but have the cats been seen by a vet in the past, e.g. vaccinations, routine physical exam, neuter/spay, etc.? Do you have a veterinary emergency clinic that you could drive to or have someone drive you to? Since you said she is not looking good physically now I would highly recommend Sushi be seen by a vet - is there a mobile veterinarian in your area who could come to you?its been a while now when my younger cat transferred this skin condition to my cat who is sushi.. i dont have a vet and vet here is too far away.. i wish that i can cure this skin condition at home because of COVID-19 maybe i can buy some stuff to heal it cause Sushi is not looking good physically right now..
You might also try my old standby, chilled chamomile tea. Purchase bags from the store, brew a cup, chill it, and swab the spot several times a day. Chamomile, used like this, is both antibacterial and antifungal. Just be sure to use commercial tea bags. Those are ALL pure German chamomile, which is medicinally active and safe for cats in prescribed doses (they can have up to 3 teaspoonfuls, 3 times a day, and more for cats over 11 pounds) internally, so the amount your cat might manage to lick off would be fine. DO NOT, unless you are an expert in herbs, use chamomile from your garden. Much of that is the English variety, which is useless medicinally, and TOXIC TO CATS!