Cat aggression toward another cat
Yesterday our youngest cat (9 months) had to go to the vet due to an emergency (accidental ingestion of prescription pill). When he came back home several hours later - our middle cat (11 months) started showing aggression by hissing/swatting at him and then at the other cat and dog in our home. It is still continuing today.
I am just looking for a way to help to soothe/comfort her so she doesn’t have this anxiety about the youngest cat who has been in the house since he was born.
I am just perplexed at how to help her. All our cats have been spayed/neutered. We take wonderful care of them providing clean litter boxes, plenty of food in a couple locations and a water fountain for them to drink out of. There are toys for them to play with.
Can you please give me some advice how to help her?
Thank you so much.