Siamese not recognising brother after being spade.

We have two Siamese cats, 1 Blue and 1 Brown. They are both from the same litre but a year apart, (brothers) At first the older one never got on with the younger brother, but they soon became friends. We have just had the younger one spade, (both are spade) and now the older one is being nasty to the younger one. It was only gone for a few hours in the morning, but when we came back the older one started to sniff the rear of the younger one and became really wary!!

Is this behaviour because there is no scent, or a different scent coming front he younger brother and the older not being able to recognise him? Why is this happening?