You could start agreeing with her and expressing the feeling that you are really sad she feels so bad? Does she suggest "doing" anything about what's wrong? If not, then a lot of commiseration and "attention" and "hoping today is not the day she dies" and that sort of thing might help. It's hard to complain/find fault if the people around you agree with you?

Did she do any little thing you loved when you were a kid? I had to take my stepmother to the doctors very early one morning so I made "sweet rolls" and bought little containers of milk and when I picked her up we first had a little "breakfast" -- the rolls were still warm and it was like a picnic and just the two of us and really comforting to both of us! If you can remember some way she cared for you and can care for her "like" that, feeding her comfort food or sitting and just really listening to her or rubbing her back (or even feet if it doesn't gross you out :-) I rub my husbands feet because he has had a nerve problem since he was 16 and only my massaging the scar helps) or something "basic" like that, might help her feel cared for a bit like your father cared for her? Maybe figure out a project you could work with her on, like going through all the old pictures and putting them in an album or something.