I think it is acknowledged that most higer order animals pine for lost comapnions or offspring, but it is doubtful if they suffer a reaction as complex and sustained as human grief.
If you mean that they were companions togetther then yes i think the animal that is left does grieve. If you are saying that your cat had a kitten that went to a new home then I am less sure.
Yes, absolutely cats grieve, they will grieve for someone that is not there that they have a strong bond with, they grieve for a fellow cat that lives with them. Cats have feelings and emotions too!