I am so sorry to hear of the loss of your other kitty. Animals can definitely grieve just like people can. It is unfortunately just going to take some time. Some things you can do to help is try working on some enrichment activities such as spreading her food out in different parts of the house so she has to find it, playing with her with toys, using a bottle with holes cut into it to put her food in (that way she has to work for it). Some of these may help distract her some and also bond more with you.
Trying to give her lots of love (if she will allow it) and extra attention will help, but it may be a few more weeks before she realizes that her sister isn't coming back.
Another option, when you yourself are ready as well, would may be getting another cat/kitten at some point. This may help her too- but I wouldn't do it too soon. She is still grieving.
If you don't notice an improvement over the next couple weeks, or if she is acting ill- not eating/drinking, vomiting/diarrhea, very lethargic, etc., I would take her to the vet to make sure she also doesn't have any underlying issues on top of everything else.
Hope this helps! Again sorry for your loss.