Feeding Amount

My sisters guy is 12.8lbs (shes also on the forum) and im trying to get him down to 10lbs. Hes a bit chunky. I had them on Costco Kirkland Signature cat food and I think that was the problem for his weight gain. Im trying to figure out how much I should feed him daily. The bag says for a 5-10lb cat to feed 1/2-2/3 cup a day. Each cup has 390 calories. He was getting about 350 calories a day on Kirkland. So I am unsure of how much to feed him I want to say 1/8 cup twice daily but im worried about depriving him of any nutrients/minerals he needs

Here are some pics

350 calories is a lot of food for him. 

Generally, feeding wet instead of dry is usually recommended for dieting, but I'm not sure about your sister's financial situation so I'll give advice regarding portion control.

Since you already know how many calories he consumes, calculate 80% of what he eats and that can be your starting point. That's around 280 calories I believe.

Also, calculating his required calories per day by using this formula: [13.6 X optimal weight in pounds] + 70 will give you an idea how many calories he should be consuming.

Investing in a baby scale and doing weekly recordings of his weight is a good idea too.

Remember to go slow, fatty liver disease is not fun. If your sister can afford wet food, I'd say go for it.

By the way, how old is he? Activity level? How often does your sister play with him?

None of the cats will touch canned food. Ive tried everything to get them to. Hes not lazy but not active hes 15 months old
 

From those pictures, he definitely looks overweight to me. As has been mentioned, weight loss is easier with wet food. Dry food usually has a lot of meaningless carbs. Cats need and will be more satisfied with a high animal protein/moderate fat/low to no carb food. If you must feed a dry food, I would recommend the best quality high animal protein food you can afford, like Wysong Epigen 90, Ziwipeak, or Young Again Zero Carb.

Regardless of the feeding guidelines, what you need to pay attention to is the number of calories fed. The general rule of thumb for indoor neutered cats is 20 calories per pound of ideal weight daily. You want to aim for 2% weight loss a week. More rapid weight loss risks hepatic lipidosis.

It is great that you want him to be a healthy weight. Obesity puts him at greater risk of diabetes. Carbs also can contribute to diabetes.

Oh, and as long as your food is nutritionally complete, and he's not losing weight too rapidly, he will be receiving the nutrients/minerals he needs.

Okay, I calculated. On Kirklands they were getting 1/2 cup a day so 221.5 calories. The one anyway was gaining weigh on that. So what should I feed? I was going to do 1/4 cup of Taste of the Wild in the morning and 1/8 cup at night that would total 130 calories a day. Or should I do 1/4 cup twice a day that would equal 195 calories daily for the two adult cats. The kitten Im feeding Iams kitten right now once this bag is done she to will be fed Taste of the Wild but I wont limit her feeding while shes growing.
 

I'd start with the 195, and then slowly cut down.

Does she feed the cats separately? If not, it might be time to try since that way you'll know how much the cats are eating actually.