Skye was 2.5kg when I took her for her injections but when she went for her spay on thursday she was only 2.3kg. The vet didn't seem worried but once she was home (and bald) I could see just now skinny she is. Her fur must be a good inch thick at least and it was obviously making her look a bit porky. I feed them both as much as they want and Skye eats like a horse but she just doesn't seem to be gaining weight.

Any ideas on how to get her weight up?

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The reason might be that Skye is not getting enough nutrition because of some reason. When you are giving her dry food, then try mixing some egg to it. The egg will provide enough protein and vitamins for him and the dry food will give him enough nutrition.
And when you are giving wet try giving beef, fish and chicken wet cat food sometimes. Try these out, hopefully she will become healthy with nice shining fur.

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Do not anticipate trouble, or worry about what may never happen. Keep in the sunlight. – Benjamin Franklin