Sister Issues...Long, but Need Advice?

I have two sisters. My older sister & I don't get along. My younger sister & I have always gotten along and been very close. For the last 4 years, they have lived in San Fran and my little sister has worked for my older sister as her nanny/housekeeper. Recently, my younger sister decided to move to Seattle to pursue a degree at Univ. of Wash. When she got here, she got depressed and wanted to go back to Cali. Both my sisters have a history of mental illness (older sister-depression, younger sister-anxiety). Now neither of them will talk to me, because they blame me for this whole thing. I'm left here wondering what the heck I did wrong. I expect this from my older sister, but not from my younger sister. I think she needs to seek some professional help to deal with her anxiety, but she refuses and now won't talk to me. What should I do? I feel so helpless.