We have two 11 year old cats from the same litter. They have always cleaned each other, slept and sit together. Last week the male cat was diagnosed with diabetes and had to spend 48 hours in the emergency pet hospital. He came home and I now give him insulin shots twice a day.

However, since he has been home, the female cat will not let him near her. If he comes within a couple of feet, she growls and hisses and if he actually approaches her, she strikes out at him.

Why this change in her behaviour and will it pass.

I have seen the same thing with my cats. My guess is that it is because the cat who has been away has a different smell as a result of having spent time in a different environment and having been handled by lots of new people. When I've encountered this change in behavior it has never lasted very long at all.

How long has it been since he returned home? Has there been any change in his behavior?

I had this happen too with my two cats which are brothers. They used to be great friends but one of them had to spend over night at the vet and when he came back the next day, the other cat would have rather of killed him if I didn't separate them.

Eventually, it will ceased when the other cat slowly but surly gets the vets smells gone. But I found a quick and working method to speed the process up. I brushed the cat that didn't go to the vet really good than with the same brush, brushed the lonely cat.
Now, their back to normal and everything is well. It took a day or so for the other cat to realize it was his friend all that time.
I hope it works for you.
Good luck