It very much depends on the temperament of the cat(s). I am afraid that generally your sister is right, although they can be taught to tolerate another cat. You need to keep a good eye on how the kitten is received though. There is bound to be a bit of biffo in the initial stages but hopefully it will settle down quickly.
Officially, lions are the only cats that live in groups but research has shown that in situations such as farms of cat rescue places where there are always more cats, certain females will make an alliance and turn into a sort of clique, looking after each other and protecting each other kittens from the others - actually, exactly as lionesses do.
Behaviourism in the domestic cat has somewhat changed from its wild form and some cats will even take a mouse or rat as a friend. It is quite possible that the kitten will wake some maternal instinct in your cat and it will start licking it and make those soft growling noises nursing queens make.
Hope things work out.