Littermates used to get along

I have a male an female. Feral kittens when I got them. Worked slow an long to get them used to me, an got there. They do only interact with me , anyone else they hide. I don't have a busy house, so this isn't a problem. I recently had to take my male to the vet for a skin problem. When I brought him home, my female acts like he's a stranger. Hissing an growling. Trying to be patient and give treats and food in sight of each other. Lil bit acts a bit better but still hisses at him. How can I get them back to grooming each other an sleeping together again? Don't want on show just need advice . They are both fixed,indoor cats and are about 4 1/2 years old
Janet Cohron
Birmingham, AL