I think you should find out more about your grandmother's past life. DOes she has trauma in her life? What kind of disappointment in life that she has experienced before? Or talk to your parents about it and ask their opinion and how to solve it. Or just invite the doctor to your house and check her conditions. Don't take her jokes or her negative comments personally and just ignore her when you heard she said something like that.

For me, my grandmother complains alot and says things like "Ah I'm stuck in this jail." She use to not be like that,i've known her for since I was little, but ever since she ended up having to live in this house cause of a car accident my mother has make her life quite a mysery,not letting her go to her counrty and that kind of stuff, yelling at her and offending her. She sits in a chair all day staring at the walls I think she has become depressed. But i'm not sure for you grandmother, for how long have you known her? Because maybe if you know how she was in the past, or of any even that happened to her to make her that way it could really help the situation.