We recently lost one of our cats and her sister keeps searching for her all over the house and meowing a lot will she settle down. Previously when we have been down to just one cat this has not been so severe.
It's always so sad when the remaining cat is so upset. Distracting her with play sessions might help. And I agree lots of love and attention are good. It's hard to know how long she'll grieve as every cat is different. Some people have found that adopting another cat cheers up the one grieving.
Do you have anything with the scent of the cat who passed? I have a kitty who is going through mourning too and is periodically searching for his sister. I read it helps to leave out items with the cat's scent to comfort the surviving cat and to help them understand the other cat is gone. From what I read, as the scent slowly disappears, it helps the surviving cat understand the other cat is gone and they stop searching.
I'm so sorry for your loss of your other kitty and I hope you find something that helps your remaining cat.