Chacha,
I'm so sorry to hear of your sister's lung cancer. Will she make it? From your words, it sounds some doubt exists.
I also gather that your sister joined the JW's when she was an adult. How deeply is she involved?
Those above factors may have something to do with whether she stays a JW or gets out at some point.
Some of us (raised in the organization) left upon becoming adults because the burden of being a JW is just too heavy. I stayed through 21 years of adult life (until age 39) when I came up against the heavy hand of Watchtower-style justice. After seeing what happened to me, I decided I could no longer support such a judgmental, hard-hearted group of people portraying themselves as followers of a loving Messiah.
If your sister is not deeply involved, you may be able to reason with her, show her information such as the book "Crisis of Conscience" by former Watchtower governing body member Ray Franz, and also the websites * http://www.freeminds.org and * http://watchtower.observer.org. These contain news and views to help honest people see the fallacy of the Watchtower doctrine.
If your sister is deeply involved with the JW's, you might want to disregard the previous paragraph for now and apply what Pomegranate said about showing "unconditional" love, love such as they cannot experience within the "ever-watchful" association of JW's.
BTW, the JW belief about the soul is that upon death, it ceases to exist. However, God would restore it during the 1,000 year reign of Christ, most likely to an earthly existence, so that individuals could get a second shot at life, but this time everlasting life. JW's believe very few Christians die and go to heaven to rule with Christ.
Hope some of this is helpful.