Cat Question
Why is my cat aggressive towards his brother after vet visit?
This is a common reaction and is known as aggression truama.
Basically the cat has been frightened by his trip to the vets and is taking his fear out / or blaming his fear on his brother.
Don't worry though, things normally sort themselves out within a few hours / days. Try not to make things worse by becoming anxious yourself over the situtaion. Remain calm and soothing around the cats and give them some distraction toys and all should be well again soon.