L.T.
I am sorry to hear about the loss and struggles your family is facing. Let them know that you love them and that you are praying for them. From my own observations sentiments like "it must have happened for a reason" or "it was God's will" are often not well-received. I have found it best to just listen. They need time to grieve, to be angry, to be sad, to vent.
In my own family we’ve dealt with miscarriages, death of my sil, and cancer diagnoses. Those of us who have talked about their feelings either with friends and family, with a counselor or with others in a support group seem to have fared better than those who have kept their feelings bottled up. We each sought out those services when we were ready to do so.
If you feel they would appreciate such a gift perhaps you could purchase a memorial keepsake - a cross or an inscribed gift. Some time ago I came across a book called "We Were Gonna Have a Baby, But We Had an Angel Instead" by Pat Schwiebert. It is written on a child's level but many parents commented that it was helpful to them. You can find it on Amazon.com with reader reviews.
You could offer to be a liason for them. They could update you on the their daughter's progress and you could make phone calls and send emails to family and friends. It would keep them from retelling the story over and over, but would keep people close to them informed.
Although I haven't lost a child my son was diagnosed with leukemia last year. The uncertainty, the fear and the sadness were overwhelming at first. We still don't know what the future holds for him, but my faith has helped me get through each day. Remembering the other good things in my life has helped as well - my son's good days, my daughter's smile, my husband having a job, having families who love us, having enough money to pay the bills, etc. Although a discussion about these things might not be appropriate at this moment, in the weeks and months to come it may help your family to focus on the other positive aspects of their life. Nothing can change the events of their lives or the pain they've endured, but their perspective on those events will affect how they heal and how they live.
I hope something here is helpful to you. I’ll keep your family in my prayers.