Girlfriends brother died, how do I help her?

My Girlfriend of 7 months brother died before I knew her. And the day he died is coming up at the end of sept.it happen 3 years ago. And she is getting really Depressed, sad, gets mad at me for little things, but then says she's sorry, she's thinking about her brother. she worned me that she gets sad around this time. But I'm not really sure how to deal with it, I understand why she gets like this but I don't know what to say because I never knew him. what do you say to some one that there brother died and I never knew him? And I live 100 miles away, so can't just be there for her have to talk to her.
Girlfriends brother died, how do I help her?
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