29 SG asked me to stop by...
It sounds like Petey is missing his brother, to state the obvious...
In my pictures area you will see Calamity Jane Grey; she was with us for most of her 21+ years. You will also see Queen Victoria Grey (Cammie's sister) and Cerridwen Blythe (Cammie's other long time companion.
Victoria and Cerridwen died within a couple of months of each other, leaving Cammie alone at 17 1/2. We waited a couple of weeks to see how she would be by herself, and then got her a kitten (Silver Kestrel the Ransacker ,-). The next year I got another kitten from the same woman with whom I worked (Hildiekatt Halfshadow the Harrier, Kessie's 1/2 sister) and then the next came Chesapeake Bandylegs the Hunter (as you can see, I am fond of excess in naming cats).
Cammie definitely perked up from having company and it took 2 strokes about 2 weeks apart to do her in...
If you do a search for Teia in the Cats Forum, you'll see my tale of rehabbing and "working in" an older cat to the existing clowder.
Teia was a rescue from the streets by the folks from whom we got her.
Persistent kindness and gentle discipline are the keys to adding any new cat to the household.
It sounds like Petey could use an older, gentle cat as a "George" to his "Lenny."
If you have a Petsmart in the area, they do a superb job of working with local shelters vis a vis kitties (Chessie came to us via Petsmart).
I know and believe that cats, being predators by nature, have more a sense of things about them than the other animals with whom we share our lives. Though dogs, being descendants of wolves, as pack animals, come close to feline emotional intuition I think.
Here's the URL for one of the photos in my Cats album...
http://www.medhelp.org/user_photos/show/180730?personal_page_id=40606&photo_collection_id=1847
Teia is on my wife's lap and Hildie is on the floor next to them.
Teia bit and scratched us when she first arrived and charged the other cats; she was a rescued street cat, still a bit feral. We took her in when her previous people lost their home to foreclosure.
Your Petey can be helped, and can have a friend again...time, patience, kindness.
Take care and let us know how Petey does.