Twinkie, I'm so sad that you and yours are going through this awful time.
As you may or may not know, I had a 26 week old female foetus (Hollie) die in utero a few years ago. So though my situation differs from yours a little, I can relate to the enormous sense of shock, loss, anger and grief your sis and her DH must be feeling. Personally I wanted everyone to leave me alone to grieve, but grief is a very individual thing, so I agree with the poster who suggested asking your sister how she would like you to handle it. Once I was out of the darkened room screaming stage, I really wanted people to talk about Hollie and not skirt around the issue.
I wish you luck, love and happiness.
WSM xxx