Daer XXXXX-XXXXX:
I am so sorry for your loss. It's amazing how much space these creatures take up in our hearts.
Everyone needs to grieve when they lose someone they love, and animals are no exception. It is best to acknowledge the fact that the remaining sister will be sad for a while. You are right on target by giving her extra attention. I found when one of my older parrots died, her mate of over twenty years was very depressed. What worked for us was tons of extra attention to the remaining bird, new toys - ones that required lots of time to figure out - and lots of love.
You might want to introduce a new sow if this one is very social, but just be prepared for her to be rejected by the older one! If you do choose to get another, check out www.guinealynx.com (forum) for the proper way to do so.
Healthy snacks would be fine also, in moderation.
My sympathy,
Dr. Lisa
Veterinarian
24 yrs of small animal vet practice. Dog, cat and parrot caretaker for 33 yrs! Former Zookeeper