jan2, pretty much the same happened to me at 21 weeks five years ago. Am so sorry to hear your sister's baby has died - my scan was an "extra" one too and the news was a total shock.
Cece has given an accurate description of the likely progression of your sister's labour and delivery - any number of pain options are available of course, more than for a live birth, and depending on what she prefers she could also have valium, which I found took the edge off the anguish. Things happened pretty much the same for us.
The sort of thing I would have found helpful to know before going in to hospital that time would have been:
Asking the staff to take bloods and swabs from me before labour started and make sure the same was done from the baby after delivery. If it is currently not clear why the baby died, these tests could provide vital clues.
Asking the staff to check the placenta after delivery carefully, and take any necessary samples too
An awareness that even with all of the above and consent to a post mortem, 50% of stillbirths post 20 weeks remain unexplained I found that so hard to live with and to be honest it still distresses me sometimes that I might have been able to prevent this happening
I found all this out from a very helpful book called When Your Baby Dies and of course it is not the sort of book you would read until you needed to
It is published by SANDS, and as CeCe says, they will be so helpful in supporting your sis through the difficult times ahead.
Like Cece I found it comforting to spend time with our ds2 after he was delivered, and the staff were so good to us. We have photos and some mementoes and a helpful present for your sis might be a special box to keep such things in. We named our son and had a funeral for him - the hospital will be able to explain what options there are, and she won't be pressurised into doing anything she is unhappy about. Even if she is not religious, the Chaplain will provide practical support.
It's great that she has such a caring and supportive sister there for her. Is this her first baby?
Thinking of you all, I'm so sorry XXX