I have had a similar experience. We raised two kittens who were brothers, then one was run over when he was two. The other cat, Simba, missed his brother and would sit by the cat flap and howl all day and all night. It was heartbreaking but after a few weeks he stared to improve and got used to being an only cat. We had Simba for another nine years after that and he was always happy, so don't feel like you have to get another cat to keep the sister happy.
If you do decide to get another kitten I would do it as soon as possible, so that she doesn't get used to being on her own, and of course, being the centre of attention!
When you do introduce the new kitten, keep the two of them separated at first, to let them get used to each other, then slowly introduce them and wait until you are sure that they are fine before leaving them alone together.
Hope that helps!
When my husband and I divorced, we had to separate our 3 cats. I took 2 and one stayed with him on the condition he get Lars a brother or sister right away. Lars gets too lonely all day long while we were at work. My ex brought home a new kitten the next day and the two didn't get along the first day, but the day after were on speaking terms and a week later as thick as thieves. I am so sorry about your kitty.