Some advice

Hi 1st time on her. I have brother and sister kittens aged 6 months, yesterday we had the boy neutered since he returned home his sister keeps growling and hissing at him. Previously they have been very close and never spend much time apart. Since I brought him home she won't go near him. We don't understand and neither does he, he seems just to want to play and be with her as normal.

This happens quite frequently after a cat has been to the vet.  Your boy's sister is smelling the odors of the vet office on her brother and does not recognize him. He doesn't smell like himself.   It's called "non-recognition aggression."  

It should go away within 24 - 48 hours but you can speed things along by swapping scents.   Rub both of them with a towel so they have each other's scents on them.  Another trick I've read about is to put a drop of vanilla on the back of the neck of each cat.    

In really extreme cases, you may need to bathe both cats but hopefully, you won't have to go this route.   

Hello nicolalizzie, welcome to TCS

Cats communicate through vocalization, body language, sight and scent. Her brother just came back from the vet after neutering and he does not smell familiar to her. That's why she growl and hiss at him.

What you could have done is have them go to the vet together and do the neutering and spaying (unless the sister is already spayed), this way they both smell the same and would not have this behavior. I'm not sure if this will help, place the brother in a room get a piece of towel and find the place where the brother used to lay down and rub the towel on that spot and thereafter rub the towel on the brother to get his scent back and to rid off the veterinary smell on him. Then throw the towel for wash. Do not use it to rub on the sister.
This is the reason I can think of why is his sister is hissing and growling at him is because he came back with veterinary smell.