You may also want to let your sister know that if she spends a little more and buys her cats a better premium food, that she won't need to feed them as much food and the bag of food will last much longer than the really cheap brands of food. The cats will get more calories and better nutrition with smaller amounts of food without all of the fillers that the cheap brands have in them.

I also loved feeding my cats Nutro Complete Care and they loved it too but now I have to feed them Royal Canin Urinary S/O diet because my cat Storm will develop crystals if I don't. My vet also told me that most indoor only cats need about 300 calories a day which isn't very much. Some may need even less.