If someone else mentioned this, my apologies, but there is an over-the-counter liquid that you can put in the cat's water. It's natural and it has calming capabilities and might help. You can find it at a pet supply store like Petsmart. I don't remember the name of it, but people working there should be able to help. I foster cats and the rescue group gave me a bottle to use when I had a very terrified feral for a while. It's not like drugs you'd get at a vet, but it helps a little. Sort of the equivalent of camomile tea for us non-cats.

This link gives some advice on how to bring a new cat into a household. Even though it's been some time, you might find some ideas there.

http://www.wikihow.com/Bring-a-Secon...-Old-Cat-Upset

One thing you might try which is somewhat addressed in the article is to contain the new cat in a room downstairs with its own litterbox, food, etc, just making sure you spend quality time with it a few times a day, but not letting it out in the house at all right now. Keep your bedroom door open and hopefully eventually your cat will see it again has domain of the house and will start to go outside the bedroom and feel safer. If you think about it, her whole space was invaded and she lost her home, in some respect, because she no longer feels it is safe. So give it back to her again, that might at least bring her out a bit.

If the other cat is upset by this, stay strong. It will all work out.

Once your older cat begins to feel safer about exploring the home again, you can start using the techniques described to see if you can get them used to each other.

One thing that wasn't addressed in the article... once they are beginning to get used to each other you can get two crates or large cat boxes, keep them in each on a bit apart but facing each other and let them get used to being in the same room but still feeling safe.

Hope some of this might help.