while I agree with the rest of the posters that inappropriate elimination can be the result of jealousy, a vet visit can't hurt.
Hitting a cat is like hitting a year old baby. She has no idea why you are hitting her, just that you are acting like a bully.
Are you letting the kitten into the bedroom where your original cat is? If you are, that may be continuing the problem. How old is the siamese cat? Some cats, especially ones who have not have lived with other cats, prefer to be only cats. The kitten that you rescued is the 'interloper', not the cat who has lived with you. If you are at the point of considering re-homing one, why would you re-home the one that has been living with you, rather than the kitten??
Here is an excellent website with a variety of links to cat care by a long term multi-cat owner. About halfway down the page, you should see links to introducing new cats into a household. (I hope that you had the rescued kitten checked by a vet to make sure she was FIV/FLeuk negative.)
Here is the link:
http://www.geocities.com/Heartland/Pointe/9352/owners.html