I agree they should be separated until he is neutered. I, too, am surprised they havent mated yet. And you want to keep them separated for 30 days after the neuter to make sure.
Neutered males can STILL mate - but they cant reproduce. So dont be surprised if you see them mating afterwards. Believe us when we say he wont care that it's his sister...
Anything to do with exotic animals is never inexpensive. Gliders are not inexpensive pets to have... Hopefully, the vet recommended to you will work out.
Let us know if you need further assistance. And...THANK YOU for not neutering him yourself.