They sound like typical sisters to me. ;o)
It sounds like Gypsy and Maggie, to be frank. Gypsy never goes downstairs, and it’s only been recently that she started leaving my parents’ room (Maggie once ambushed her or something, and Gypsy freaked out).
Most cats I’ve had generally just tolerate each other. With the exception of Annie and Luci, none of them are really the best of friends. Annie and Luci are litter mates, but even then, Gypsy and Buffy are too, and even then, Gypsy doesn’t like Buffy. And Maggie doesn’t like ANYBODY.
Hey – my old cat, Fluffy never went in the living room after we got the dog – she knew that was the dog’s space. Animals are weird that way.
Are they attacking each other? If not, give it some time, and don’t force them. Cats ARE territorial. (At least mine are.)
Thanks for all the input. Unless they become friendly toward each other in the next week or so, I’m going to re-home Gem. I’m just not willing to keep animals in my house that don’t like each other, and it doesn’t sound like that is likely to change, so Gem’s going to find a new home.
This smacks of urban legend to me, but if you want to give it a shot, go ahead:
Put a teaspoon of cod liver oil on the back of the cats’ necks, then lock them in the bathtub together. As they lick the oil off each other, they will then become good friends.
And if they don’t, you have the joy of trying to wash the fish smell off two cat’s heads. 
An amusing idea, though…I’d definitely want to make sure they both liked cod liver oil first.