Cats don’t remember each other

This may sound dumb or not, I don’t know. But I have 2 cats who are sibling and they were adopted at the same time 3 years ago. Recently the male, Spencer, got sick and had to stay at the vet for a week. He is better now and has returned home but his sister, aspen, keeps hissing at him which she never did. She acts like she doesn’t remember him and I am worried about it. Is there anything we can do to help? We thought it was a smell thing because he didn’t smell like home but it has been a few days and she still hisses at him. He doesn’t his at her though.