slight problem

we got our boy cat fixed and when we brought him home his sister and our cat hiss at him but the mother cat (who was spayed a month ago) doesnt.

he is 3 months old and weighs 4.5 lbs..some what big?

My guess, its the unfamiliar smell of the vet's office for the sibling. Mommy still remembers her baby though. As far as the size, I'm not sure!

how long will this last?

Put a drop of vanilla extract on all of thier necks, then they'll all smell the same. That's the advice I've seen given alot. If that doesn't work, try a Feliway diffuser (from the pet store). I'm sure others will be along to give more advice.

Its very normal when you take one cat to a vet that he come home smelling differently ....... they just dont recognize him at the moment...... Just take a towel and rub on the cats that stayed home ...and then rub on the cat that went to the vet ....... he will start smelling familiar to them ........

When it has happened to me ...... it usually takes about 24 hours ........

the weight question .... Im not sure about ..... sorry

It can last anything from 24 hours to a couple of weeks. When Mosi came back from the vets after his neuter, Jaffa was very hissy with him and it took at least a couple of weeks for things to get back to normal.

well my ctas are older and they probably weigh about that much...maybe a little less...nothing to worry about I'm sure. But just to make sure, make sure hes on a healthy diet and isnt eating too much. talk to your vet about it. Im not a vet so i dont know if the advice I'm giving you is completely correct but its worth a try!