It's always a real good idea to isolate the two cats for a while and only gradually introduce them to each other. Cats can be very jealous and possessive of their humans! It's even advised by some that it's best to not give the new cat any attention while the older cat is watching. Make sure the older cat knows her place is secure. You may have to repair some hurt feelings at this point! but I bet if you put some energy into reassuring your older cat, it will help. Finding a way to keep them separate for a while and only introduce them gradually, through a screen or perhaps keeping the new one in a carrier with a cage front on it, so they can get used to each other's scent for a bit... that has worked for me in the past.
