my cats behaviour

Question
My kitten just had a stomach surgeory for removing an object she ate :(
When she was gone her sister kitten and our dog seemed to go on with life...  She was really dehydrated when she went to her surgery as she is a small cat 2 days of not being sure what was wrong with her... thinking she just ate somethign since she ate my breakfast the day b4 that when i left to get a drink..  we thought it was an upset stomach. It dehydrated her the vet told us she would have prob died the next day .. But thingsturned out well 4 days later she is back home with us!!  and a huge vet bill and a stressful time .. later the vet told us when I told her they didnt even seem to act weird when she was gone just went on.. she told us dont be surprised if they reject her for a bit.. ignore her.. when she comes home..
 All day long her sister who was extremely close to her B4
has been hiding and hissing at her all day long.

Why is this happening?
How can we make her realize its Faith or to get her to stop hissing at her.
When Faith got home from surgery she was just running over to her sister and did not get a good homecoming response.

Answer
Chairity,

I'm sorry about your kitty, that's a terrible thing for the kitten to go through, and that is normal behavior towards cats that come home from the vet.

Cats go by smell and not by sight in recognizing other cats (with humans, they go by voice...not by sight). Your kitten brought home MANY different smells from the vet: anesthetic, medicinal smells, and smells of other cats...all clinging to her fur and skin. The other animals thinks she's another cat because she doesn't smell the same! Things will settle down  when the smells are gone from Faith.

As Faith grooms herself she will be replacing her regular scent. You can keep them separated if you want until the smells are gone from her fur. It will take a few days.

You can also rub the cat who did not go to the vet with a towel, then rub the cat who did go to the vet with the same towel to transfer familiar smells but do not do it the opposite way because you will then be transferring the vet's office smells to the other cat.

A trick that I've heard a lot of people use in that situation (though I have never tried it) is to put a drop of vanilla on each cats forehead so they smell the same.

Try giving both cats a special treat, like a can of tuna. Feed them next to each other so that being together is a pleasant experience and not a negative one.

There is cat calming product that you can add to the food or water that reduces anxiety called Bach's Rescue Remedy. It is available on-line and in health food stores. Here is a link about it: (copy and paste, or type the whole links into your address bar)
http://www.bachflower.com/Pets.htm

Things will get back to normal once the vet smells are gone from Faith and the other kitten 'recognizes' her.

Tabbi