“My 1 1/2 yr. old Burmese is, or was, quite laid back. Three months ago I found an 8-week old abandoned kitten and brought him home. My Burmese was not pleased, at first, but after about 3 days, they became fast friends. My male Burmese cleaned him and slept with him and taught him all of his devious ways. But, today, I took them both to their Vet. My Burmese was due for his yearly check-up and the kitten was due for his 3rd round of kitten shots. Since we came home, my Burmese has been very aggressive toward his "little brother". He is hissing and screaming at him. He wants nothing to do with him. Baby brother is perplexed and so am I . ”
One of my friends uses a peace of dirty laundry. In this case since both cats have been to the vet you can wipe each cat down with different prices of dirty laundry. This will make them both smell like you instead of the vets office.
It could just be that the older one was upset about going to the vet or if he got any annual shots hes not feeling well from them. I wouldn't be worried unless the behavior continues past a few days.