How to be supportive when someone is gloomy...

In the past, my older sister has had ups and downs in her disposition, and sometimes sinking into depression.

She has been remarkably upbeat the last few years, even though her husband died five years ago. Yesterday I called her, and the anniversary of her husband's death is this week and she was feeling very low.

Although I understand this, I find she ventures off into other tangents... mum always liked you more than me... I had the burden of helping to raise you all when Dad was ill and Mum was working...I never had any friends growing up, etc.

We are around 3 years apart in age, and raised in the same environment. It seems all her life she found the cup half empty whereas I have always found it half full, if you know what I mean. I tend to be more upbeat in nature.

I still don't really know how to react after all these years. If I tell her to find some joy in everyday life, she knocks that down. I cannot convince her she is forgetting the joyful things about growing up; she only remembers the bad. I try to change the subject to something more cheerful; she brings it back to past grievances. I ended up cutting the conversation short with an excuse about a roast in the oven, but felt bad afterwards.

How can I boost her up a bit when it seems she resists any attempt?