When cat comes back from the vet?

We have 2 cats, 1 for almost 2 years. The other we just got about 2 months ago. They have never been friends. The existing cat Nisha is not a friendly cat anyway an bring in the new one has not been fun for her. She resents her terribly. The new cat Sammie came into heat ( we thought she had been spayed ) while she was in heat Nisha was even more aggressive towards her. Now, we have taken Sammie to the vet to be spayed. When she returns I am sure the hissing, growling, swating will start again.
Any advice that I can do for the new cat an Nisha to help calm things?
I would so appreciate any advice
Thanks so much

When I brought SImon home from an extended stay at the vet, my other cat, Max, didn't want anything to do with him. A tip I got from here worked wonders. Put a drop of vanilla on each of the cats. Under the chin, between the shoulderblades and on the back near the tail. This way they both smell the same. That should help some.

I would re-introduce them to each other, slowly. They may never be the best of friends, but this might help reduce the hissing/growling/etc. There is a tread somewhere. I'm sure one of the mods will be able to find it.

Another trick is to take a towel and rub down both cats. That helps to transfer smells.

Thanks so much for the replies, I will try them.. I intend to reintroduce because I feel with the new cat just having surg she will need alone time away from Nisha.
Then I will try the vinilla.