Sometimes we need to supply a voice to the thoughts in ours heads and I'm glad you did it on here. I'm terribly sorry for the loss of your sister, she sounds like a terrific person. Are there any coping mechanisms you've used in the past to ease your depression?
Right now, I have to set goals for myself (that'd normally be ridiculous). Like, right now I'm forcing myself to stay out of bed for a certain number of hours per day. If I don't, then my depression gets worse. Maybe small goals could assist you with your depression. I hope your cat feels much better and can cuddle with you during this time.