Even though they are siblings, it's very possible that they had different fathers. Many kittens from the same litter don't even look alike and they can also have very different personalities. I currently feed my five cats Innova Evo which is a grain free high protein food. It's high in calories so my cats don't need to eat as much of it as other brands. An overweight cat on a high protein will lose weight as long as they aren't free fed this food. Mine only eat about 1/2 cup of dry and they also get some canned Innova Evo too. If I did free feed them then Ziggy and Sky would eat too much they'd gain too much weight because they love to eat. Good luck.