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Hi, I am Dr. Peter and would like to help.
1- Is he drinking more than usual?
2- How long has this been going on?
When a cat eats more than normal or has an increase in appetite, there are several conditions associated with this like hyperthyroidism (over active thyroid), diabetes, sometimes kidney or liver disease, etc. But, the most common is having an over active thyroid as your sister suspected. Having an over active thyroid can predispose to other problems like kidney or heart disease if the thyroid is not controlled. Your cat needs a hands on exam by your veterinarian and a blood test to rule out an over active thyroid. There are medications that can make his thyroid normal again. Follow up with your veterinarian.
If you have additional questions I will be glad to answer them.