Would my cat know another sister was not the original sister he had?

I think the cat would know the new kitten is not the original because, as you mention, of the scent. I think many animals use smell more than sight.

He would know the difference.

I don’t think he’d believe it was his sister. If he is used to having another kitty around, though, they might get along very well. How old is CK? He might look at a new kitten as a “fathering” opportunity.

Yes, he would know. Wouldn’t you? haha

Usually introducing new cats is pretty easy in the first few years. If CK is under age 3 he will probably take to a new pal just fine, of course you should give it at least several weeks for him to adjust, he will probably be at least a little pissy in the early days. Cats are very jealous creatures.

Animals don’t recognize family relationships like we do. He’d recognize her as another cat at a certain maturity and with a distinct smell. Unlikely he’d pay any attention at all to her color or markings. He’d adjust to just like he would to any other cat of that age and gender.