Post Spay Sibling Issues

I have 2 five month old kittens, a brother and sister. Brother got fixed about a month ago, but our little girl just got fixed yesterday. They are typically quite playful and even a little rough with each other from time to time, but now I can't get him to leave her alone. As soon as I go in another room, I hear her getting attacked. I try to separate them (did this one night when he was neutered too), but they will cry ALL night long for the other kitten. Neither of them like to be separated from each other. Any other suggestions for getting him to leave her a lone a little bit? It wasn't too much of a problem with his surgery because she's not as rough as he is and he also bounced back so quickly -- was still attacking her with his come on! Lol. Thanks for any suggestions!!

He may be reacting to the different smell (vet, lower hormone) of his sister.
You could try dabbing a little vanilla on each of them so they both smell the same.