Neuter reaction from sister cat

I just brought my 6 month old cat home from getting neutered, and now my other cat, his litter mate, will not have anything to do with him. She hisses, swipes at him, and generally makes him miserable ever since he got home, and they were inseparable before the operation. I've never seen this kind of reaction before from a neuter, is this normal? Will his sister eventually get used to him again (he was gone only a half day) and they'll start to play like before? My male cat seems very distraught by the fact that his sister won't have anything to do with him.

[Aleda M Cheng, D.V.M., C.V.A]

His sister may be reacting to the smells of the surgical scrub, blood, and medications.  These will wear off in a week or two and things should go back to normal.