Please don't pity your sister, but also recognize that she is going through major stress and loss at this time. Be extra gentle and understanding.
I loved pictures of me. A dear friend came the day after I had her and took pictures of me and my baby. She got wonderful pictures. I don't even mind that my hair is a mess and my face is red from the duramorph. I was so happy she was alive. However, she called me before she came, made sure it was ok to come, and brought her camera. She helped me get mascara on as my doped up hands could not handle that. (I am sure the mascara made me look SO much better.)
Don't quote stats. I hated that. I knew them already. If she is happy, DON'T rain on her parade. Don't talk about friend's babies that died. Don't talk about other babies who were sicker. At that moment, your sister's baby is the sickest in the world, to her.
Don't look at the other families, unless they talk to you. You don't know the "manners" of the NICU. One grandma got in, and started to take a tour. I was holding my little one, and she started over to see my baby. I am sure my body language turned her off, so she headed over to look at the baby next to ours. He was a very sick 25 weeker. I told her to go find her own baby and leave ours alone. (His mommy and I were NICU buddies.) I then called loudly for the nurse. Ignore all the other babies. Don't ask questions. There is an unspoken contract in the NICU that you ignore the other families unless they talk to you, at least in my experience. (It's different for the parents, I am talking about guests here.)
She may only get to hold the baby a little bit. I would suggest that unless she offers, don't ask to hold the baby.
Bring her a small treat. Bring candy for the nurses. For sanitary reasons, I would suggest individually wrapped ones. Chocolate went over huge, as did sugar free candy. I just put them in a small gift bag. A little bribery never hurts.
Don't forget her husband/partner. Bring him something.
I don't know your relationship with your sister, but I WANTED people to come see my baby. I wanted to have people in there within limits. So much of having a new baby is a social/family event, she may want this. I just loved it when someone came in and cooed over my baby. If you or your child is sick, don't visit then, it's not worth it. Sometimes I just wanted to be left alone, so respect that. We were only allowed 2 visitors, so if someone was in there, either I or DH was out.
Even if you are not ready to visit inside the NICU, perhaps take her to lunch in the cafeteria, or even better idea somewhere else. I would go days in there, and the sun felt WONDERFUL on my skin when I went outside.
Send her a congratulation card and a gift. Another dear friend purchased me a subscription to UsWeekly as she knows I LOVE tabloids. Every week when it comes, I remember how she loves me as a person. It was also very nice to have fluff reading material. When I was finished, I passed it on to the nurses.