Why does this happen?
This is fairly common and is termed Non-Recognition Aggression, cats rely on scent to recognize each other. Her visit to the hospital has changed her scent to the point where her brother can no longer recognize. A few theories behind it are:
- The cat expressed his/her anal glands or is emitting pheromones associated with stress.
- The cat took home scents of other animals who were previously at the clinic
What to do?
It is a good idea to keep them separated in the meantime for each others protection. A few things that can help:
- Giving blankets with each others scent to opposite cats (i.e brothers blanket goes with sister and vice-versa)
- Feliway Friends
After 24 hours try to gradually re-introduce them, either by opening the door a crack for them to sniff or to have a gate in between. It does self-resolve after some time you just need to make sure the conflict is over before putting them in the same room.
How can I prevent this?
Next time, if possible, bring both cats to the hospital - they won't lose sight of each other.