My older cat will not accept my new kitten!

Have you tried letting the new kitten have her own space within the house that is not occupied by the 12 year old and then introducing them slowly?

I found this link that has some very good suggestions for introducing two cats. I hope it helps!

http://http://www.littlebigcat.com/?...tintroductions

It takes time. Put one in a room and let the other one smell her under the door. After a few days, open the door and put a baby gate across the doorway and let them look at each other. Reassure the older cat as she hisses through the gate at the littler one. It's pretty much just a matter of letting the older cat know you still love her.

Okay, the rule when introducing a new cat to older existing ones is to do it one sense at a time. The longer you take the better chance you have at them getting along. Just start over!
From About.com
* Set up a comfortable "safe room" for New Cat. Put her food, water, litter box (not near the food), scratching post, toys, and bed or other sleeping mat there.
* Expect a great deal of "hissy-spitty" behavior through the closed door from both cats. This is natural and normal; they are just starting to explore their "pecking order."
* Scent is very important for cats. Let each of them smell the other indirectly, by rubbing a towel on one and letting the other smell it. They will soon accept the scent as a normal part of the house.
* Once or twice, switch roles. Put New Cat in the normal living quarters, and let your resident cat sniff out the new cat's Safe Room.
* After a day or so, let the two cats sniff each other through a baby-gate or through a barely-opened door. Gauge the rate at which they seem to be acclimating to each other.
* When you think they're ready, let them mingle under your supervision. Ignore hissing and growling, but you may have to intervene if a physical battle breaks out. Again, take this step slowly, depending on how quickly they get along. If they do seem to tolerate each other, even begrudgingly, praise both of them profusely.
* Make their first activities together enjoyable ones so they will learn to associate pleasure with the presence of the other cat. Feeding (with their own separate dishes), playing, and petting. Keep up with the praise.
* If things start going badly, separate them again, and then start where you left off. If one cat seems to consistently be the aggressor, give her some "time out," then try again a little bit later.

The introduction can take from two hours to six months, so don't be discouraged if your cats don't seem to get along well at first. Often the case is that they will eventually be "best buddies."
Good Luck

I'm having the same problem. I have a female orange tabby that is a year and a few months old and recently adopted another female cat that is almost a year old. My new cat keeps trying to play with my cat and my cat keeps getting freaked out and hissing and running away like a bat out of Hell. *sigh*