E.A.
I don't know if this is exactly what you're looking for, but "When bad things happen to good people" by Harold Kushner is an excellent book for dealing with tragedy. What I liked most about it (and why I recommend it to people) is that it doesn't try to force a painful event into happening for a reason (ie - "this is all part of God's plan") so that a person going through a stressful time, like your sister, doesn't feel burdened or guilty when she wishes that this had never happened to her fiance. Kushner is a Jewish rabbi but pulls from many religious traditions in his writings.