Sudden hostility

My Teasel and Ivy are 2 year old DSH, adopted from the same litter. They're best buddies, sleeping together, playing, and grooming each other, up until two days ago. That's when we took Ivy to the vet to have her checked for urinary tract problems. (she's fine, just picky about her litter box) They kept her there for a few hours, until they could get a urine sample. When we brought her home, Teasel started hissing and growling at her. We figured it was because she smelled different, due to hanging out at the vet's office. It's been several days, the carrier has been washed and put away, and Ivy has gone back to her normal routine. Teasel, on the other hand, is still being a jerk. He won't let Ivy eat in the same room, and growls whenever she is nearby. She doesn't seem to notice anything's wrong, except that when she tries to play with her brother, he gets REALLY mad.

Does anyone have any tips for helping them get along?
Better yet, any tip on how we can prevent this from happening next time we go to the vet?

Welcome to TCS.

The answer to both of your questions, simply, is yes.

You are absolutely right - the problem is her smell. She smells of "vet." Many cats are sensitive to this. It usually self-resolves within a few days - but it would seem that Teasel is VERY senitive to this.

Cats have very sensitive noses - and while the crate is put away - he can obviously still smell "vet visit" on her.

Here's the usual advice. Go to a health food store and purchase real vanilla. (Yes, the liquid stuff you bake with!) Don't use the supermarket synthetic vanilla.

Use it on BOTH kitties. Dab it behind the ears, at the base of the neck, at the base of the tail, on the butt, and most importantly, under the chin. Make sure you get a good dab under the chin - this wafts up into their nose.

This helps confuse the scents, and things settle down a lot more quickly.

In this case, I would recommend using some pet wipes on Ivy first (may help rid her of the vet scent) and then "vanilla" both cats.

In the future, just vanilla both cats as soon as the vet-visit kitty comes home.

Man you guys here know a LOT!! I wish I had found this place when I first adopted my kid...

Are you 100% certain that Ivy is the one peeing outside the box and that it couldn't be Teasel? -or Ivy's doing it because Teasel did first? Just curious because if he was the one with UT problems that could affect his behavior, too.