My sister is manic/depressive. I knew that for years as she had all the classic symptoms, but she was either undiagnosed or in denial. About a year ago she called me up and said "Wow, I just got diagnosed as manic/depressive. I never knew" I literally said "Well, duh, of course you are".
She is on meds now and the difference is lovely. We talk daily, she is supportive when I need her to be, I have a sister that I never knew before. Before the diagnosis it was a chore to talk to her. It was always about her, her problems, her everything. Once I got "Hi, how are you" out, there was no dialogue in our conversation. So, I limited the time I spent talking to her. Which was sad, but had to be done for my own sanity. Oh, we still talk about our issues, but now we talk about OUR issues, not just HER issues. Plus, I really don't miss the hysterical crying jags - and neither does she. :)
Encourage your sister to go to therapy. Mine does and it is helping her. We make our own reality and if your sister is only focusing on the negative she will never make a happy reality for herself. Negativity breeds negativity. But, all you can do is encourage and guide and try to lead by example. If she starts complaining about work, you counter with "Oh, but you have such a great job". If she complains about her neighbors, you counter with "Oh, but you live in such a beautiful home", etc. Share recipes that "you love" but that are low calorie and heart healthy, talk about positive things only. Have you ever told her, outright, that she is negative and it drives you crazy?
But know that, ultimately, only she can change the way she behaves. You can only control your reaction to her.
Good Luck and God Bless