Cat suddenly hissing at brother

My two 3 year old cats are litter mates and have always gotten along swimmingly; they play/snooze together and groom each other all the time. However, even though they're the sweetest cats at home, they turn into absolute terrors at the vet. I've never seen anything like it and I had a ton of cats growing up (imagine the exorcist noises times 100). I'm convinced they remember being neutered and have never gotten over it, but that's neither here nor there.

Anyway, I had to take one cat to the vet yesterday because he had some gunk in his eyes and I wanted to make sure everything was ok. The vet said he seemed fine but is doing some blood work just in case (results come in on Thurs). Ever since the one cat returned from the vet, the other, his brother, hisses every time he comes near him. At first I thought the "vet cat" simply still had the "vet smell" on him so I rubbed them both my stinky clothes to get the scent off. But now, well over 24 hours later, the cat who stayed home still hisses furiously when his brother comes near him. Any thoughts on what's going on?? Do you think this will pass? I'm so worried they'll no longer be the cute cuddly litter mates they used to be

He probably still smells funny from the vet. Try putting some vanilla extract under both cats' chins and near the base of their tails. This will make them smell similar. Make sure it is real extract and not the artificial stuff.

From the time she was a kitten, Lucy adored Much but when Much had to have some surgery, Lucy didn't recognize her at all and it took a week before she did.

If you continue to have problems, then look into getting some Feliway diffusers and do some re-introductions.

When they are ill they give off a different scent, the other probably recognizes this and is reacting in a freaked out way, doesn't recognize this smell........Keep an eye on them and when he gets better the other will chill out......