Sometimes a cat comes back from the vet with a "vet smell". The other cat
doesn't recognize their scent and it takes some time til they reestablish
the familiar scents and recognize each other.. Often they just take a little
time and readjust to each other on their own, but sometimes you might need
to rub a towel on both of them to mix their scents, or try a product called
Feliway. Let us know how it goes with them!
Judy F

Smell. Cats work on smells and he probably smells on iodine/some other drug
used in opperation, as well as the other smells he picked up at vets,
probably even the smell of cast. Cats don;t depend on sense of sight as much
as humans do. So probably she thinks he is an intruder on her territory
rather then her beloved brother.

You can try and give him a gentle spunge bath, but you can;t really touch
the operation site for at least a day or two. Perhaps rub a touch of tuna
brine onto him, so he smells more "pleasant" to her. But even if you don;t
do anything, he will soon pick up the home smells, just give them some time
and let them reaccoint themselves to each other without much interfierance.

Gee

Lewis at Lewis@alexhotel.co.uk wrote on2/5/04 8:37 AM:

He smelled like the vet's office and like his anesthetic when he came home.
This made him seem to his sister to be a strange cat, rather than himself.
Typically, the situation resolves all by itself, so just let them work it
out , so long as there is no severe fighting going on.  The cast might be
making the process take longer, since it may have an inherent strange odor,
as well as changing his appearance.  Most cats find their way out of a cast
as quickly as they can, so this part of the problem may go away sooner that
you would like.  How in the world did he hurt his leg, btw?

He probably still smells like the dreaded vet's office.  Take
something that your girl has laid on and rub it all over him,
especially over the cast.  This should take some of the strange smells
off of him.  

Nan

As others have indicated, it's probably the smell from the vet's office
lingering on.  Or, it could be a sign that she's getting ready to go into
heat. You don't indicate whether or not his sister is spayed yet, so take
the opportunity to take her in and have her coming home smelling of vet's
office as well.  They can both adjust at the same time.  At 10 months if she
isn't already spayed, she very well could already be pregnant, and the most
likely father would be her brother.  I'm sure you don't want THAT
complication, so go ahead and fix her too.

Thanks to every one for help.Not sure how he broke-dislocated his leg.
It looked as if he had caught it in something(one down eight to go ) .
BTW sister was spayed three months ago.

Smell - cats recognize by smell more so than sight.  The cat that just came
home smells different.  It can take a week or a month before they adjust.  I
have one who has had numerous vet visits, even just a short one and one of
his "brothers" has a fit and won't go near him for weeks.