As with any loss, both they and the other household members adjust to a new dynamic. Sounds like Mona has done just that. Still misses hers sweet sister. I can see how bonded they were.
Mona and her litter mate (Lisa) came into our family 12 years ago. I wanted a Tuxedo and heard about 2 kittens rescued and staying @ a vets office. I went to see her and she and her sister came bouncing out of the crate. That did it I just couldn't split them up and took both home. It's been a real hoot and I've been glad I had both. However we lost her sister Lisa in June. I was afraid she would be lonely, but she has been coming into her own. Lisa was the dominant kitty and now Mona enjoys being the top cat. She stills roams around the house looking and meowing. Mona has become the lap cat her sister used to be. I don't think I will get another kitty. Mona is finding life pretty good. I will post a profile about Lisa.