Sister has turned aggressive!

Question
We have two female cats, from the same litter, approx. 11.5 years old.  We got them when they were 6 mos. old.  Hobbs is the more aggressive and independent one, preferring outdoors, playing, etc.  Blackjack has always been nervous (hides under the bed when the doorbell rings, etc.)and curious.  Five years ago, we took in an old, male cat (he showed up and refused to leave!).  He was very gentle and obviously very old.  Like the girls, we took him to the vet regularly. Hobbs did not like Chuck, so she stayed outside even more.  Three months ago, Chuck died.  Since then, Hobbs and been indoors much more, which has been nice.  However, she is starting to terrorize her sister.  She has always been playful (jumping, chasing, trying to engage Blackjack in play), but now it is mean.  She jumps Blackjack when BJ tries to go in or out the kitty door to the garage (where the litter box is), or jumps her "wherever".  Blackjack is a nervous wreck.  She is afraid to go out the kitty door and has started pooping in the house.  (We had a litterbox indoors for Chuck the last year because his health was failing and he couldn't maneuver the kitty door, plus he would get confused, etc.  We had to re-train Blackjack to use the box in the garage and just when we succeeded, Hobbs has fouled up the process!)  Is Hobbs just trying to assert herself as the "top cat" now that Chuck is gone?  Will she calm down?  What can be done?  It's more than the litterbox, although we hate to bring one back indoors.  Blackjack spends all her time in our bedroom, (a lot of it under the bed) one place Hobbs never ventures.  She is missing our companionship and we miss hers!  Thanks.

Answer
Ellen,

Anytime a cat's behavior changes noticeably you should have the cat seen by a veterinarian. Expecially at 11 years old (your cats are approx. 62 years old in human years) they should have an "elderly cat checkup" to check the function of ttheir liver, kidneys, thyroid, etc.

Hobbs may be in pain somewhere which would make her aggressive, it could be her personality is changing because of her age (cat's get a form of kitty alzheimer's or senility, though your cat's not THAT old), she could have separation
anxiety because of the death of Chuck (maybe she liked him more than you realized) and she is acting out, or yes, she could be exerting her dominance and reclaiming what she thinks is her territory.

You might try Feliway. It's a spray of something like kitty pheronomes and calms cats down. It is very popular and highly recommended. Some people even spray cages with it 20 minutes before taking the cats in a vehicle and it keeps them calm. I think you can get it at PetSmart or from a vet. Also the vet can give the cat a short term (or long term if needed) precscription for "kitty Prozac" to help calm Hobbs down. That is becoming popular for behavior modification.

As far as Blackjack is concerned I would make it easy on her. Older cats do not take stress as well as younger cats. It  can permanently affect their personality and even make them ill. You may want to get her some "kitty Prozac" to help keep her nervousness about Hobbs.

Suggestion: I know it not something you want to do but for Blackjack's peace of mind maybe put a litterbox in the room where she prefers to hide, or if you have an attached bathroom you can put it in there to save her the stress of having to use the kitty door and be attacked by Hobbs. Maybe even put her food and water there. Put Hobbs outside or somewhere she has to stay put then bring Blackjack out and give her LOTS of love and attention plus some kitty treats. If you have TV in the bedroom, spend more time watching it there to keep Blackjack company. You will need to make some concessions as the cats get older.

I am attaching a link to some good articles about older cats just for your future information (copy and paste or type the whole link into your address bar):

http://www.sniksnak.com/resources/geriatric.html

I hope this helps.
Tabbi