I'm so sorry for your sister's loss. She's lucky to have a sister like you who would come on here and ask the question.
I didn't have to make that kind of decision, but did birth naturally and I would say just make sure someone is there for her when it all happens - tiny premmie labour is very different to normal labour and it may not be recognisable as what it is until her body is pushing her baby out, though I was in denial, so that may have been why, but it can all happen pretty quickly when they are that small. I won't lie, it is hideous to feel your body pushing out a baby you don't want it to and fighting it may may the pain worse, so if you can just help to relax her that may be better. Again my situation was a bit different as I knew at least one of my babies was still alive, I'm not sure what happens in this type of case, so sorry if that's not the right advice.
Will she be in hospital? If so make sure she has some gas and air or similar pain relief there just to make it a bit easier. The nurses may also be able to do hand and foot prints for her too, make sure this happens if possible, and try to gather as many keepsakes as possible, even if your sis doesn't think she wants them, as she will probably at some point and better to have them and not want them than the other way round. I agree with the other poster who said take pics as well, and encourage her to spend time with the baby if she is able, it may seem macabre, but it really isn't at athe time and does help a bit.
Maybe take over the practical bits for her - if there is a special blanket or something she'd like the baby to be wrapped in (it will be really tiny, about maybe double the length of your hand at most), you could bring that, and help her with the other things afterwards. We had a wee private gathering at the funeral directors with friends and family and released balloons when they were driven off to the crematorium, it may really help to arrange some sort of send off or funeral, so you may be able to help with that - the hospital often has some contacts with funeral directors and many will do lots of it for no charge or minimal fees. The worst feeling is walking away and leaving your baby behind, so at least if something is in motion for a goodbye then it feels a little better. Then just be there for her and listen to her, it will be really hard for all of you. I hope it goes as well as possible for her and I wish you all the strength in the world in your support for her.
So sorry again. xxx