A. Have her seen by your vet. At her age and with her symptoms you need to have your vet rule in/out hyperthyroidism. This is done via blood work. A CBC, chemistry panel thyroid panel and urinalysis can be submitted to assess major organ function.
A. I can't speculate on why her sibling died last year (and I'm sorry about your losing your pet). It might seem that the two of them have some type of genetic problem, but I doubt that the kidney failure in the first cat is related to the signs you are seeing in your second cat. I think that you just have some bad luck going on, and as both cats are/were well into what we consider to be "geriatric" years, you're going to start seeing some problems, unfortunately, speaking from a statistical standpoint.
Knowing what caused the renal failure would have taken some diagnostic work at the time, including an ultrasound to visualize the kidneys as well as (of course blood work) and a thorough history. Sometimes we never find out what causes kidney failure, but an ultrasound could have helped us understand if there was a congenital (present from birth) problem that caused the ultimate failure.
As for your remaining cat, I think the previous answer is spot on. The first job is to diagnose what is causing the increased appetite with weight loss, and the number one cause of those symptoms in 12 year old cats is hyperthyroidism. This is a metabolic disease that usually involves a benign growth on the thyroid that's causing overproduction of thyroid hormone. This results in a ramped-up metabolic state, causing hunger and weight loss (and lots of other things, like vomiting, diarrhea, and an elevated heart rate).
Hyperthyroidism can be treated successfully, but you need to get her diagnosed in order to do so. Other things to rule out are a problem in the intestines that prevents absorption of food, but that's less common than hyperthyroidism.
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