DD1 was born - unexpectedly - at 32 weeks and was in NICU for 23 days. In the beginning she had no sucking reflex and was fed through a nasal tube., In the early days she was only taking 2ml of my expressed milk at a feed - also my milk did not come in properly as labour was so quick...
I do think it is poor that there is not room for your sister to stay with the baby. When DD1 was born I stayed in the maternity unit for 3 days but then came home. Our NICU does not have parents suites or anything (although they are rebuilding and getting them.) I was back and forth to the hospital all day from the first feed to the last - about 3 miles each way.
I did find, and your sister may experience this too that, whilst being very supportive, the NICU staff are more focussed on the baby gaining weight and measuring how much milk she was getting. They provided me with a fabulous expressing machine and I felt very cow-like for weeks! If I wasn't feeding I was expressing and so on.
DD2 also came early and I experienced many of the same issues regarding breast feeding. She was born at 35 weeks and only in NICU for 10 days. I tried even harder to get the breast feeding established and they were helpful and supportive.
In the long run, with both girls I managed to give them some EBM and some BM until their due dates. I struggled to make enough milk even with medication to help. Neither girl ever really got the hang of sucking - partly due, I feel to having been tube and then bottle fed as the focus was weight gain.
With regards to kicking up a fuss, I know you said sister's husband is busy with other DC but can he not kick up the fuss instead of her. She must be feeling fragile enough and that is before her tiny baby arrives.
Also NICU/SCBU here had/have open visiting for parents (and siblings) of their babies. I really did practically live there with DD1 beyond coming home to sleep. They were happy for me to turn up at any time - day or night - and I did turn up in the middle of the night when I felt anxious or just wanted to check on my baby (or tiny baby rabbit/alien!!!) in the early hours of the morning. We also phoned a lot when we were not actually there and no one ever gave us any inkling that we were being a nuisance.
Your sister will probably settle into a routine surprisingly quickly - whether she gets to stay in the hospital or not - although do be aware that when NICU here was short staffed or very busy with babies they would close visiting to anyone other than immediate family - so you may not get to see the baby until they can bring her/him home.
Good luck to your sister - oh, and my girls (DD1 is 3.1 yo and DD2 is 10 mo) are fighting fit and perfectly healthy - which was not the original prognosis with DD1.