As the mother of a 26-weeker who is currently nearly 7 weeks old and just out of NICU (but still has a long way to go before home), personally I loved people sending small baby clothes and soft toys (they can go in the incubator), because it felt like a positive 'welcome to the world' kind of message, even though things were terribly uncertain at that point - but I guess some people might find that difficult. The picture will probably be a little clearer in a couple of days.
Visiting - be guided by her. Personally I haven't wanted anyone but immediate family at the hospital, it's too much of an infection risk. And yes, unfortunately, it'll probably be difficult for her to see you with bumps, as it'll be such a direct reminder of what she's lost.
In terms of practical help, try not to say to her 'oh do let me know what I can do' - that puts the onus on her, and she has no spare energy to think up useful things for you to do. What has really been brilliant for us over the past few months is people doing helpful things on their own initiative that require no input from us - eg sending an emergency package of homemade chocolate brownies, a casserole, that kind of thing.
Regardless of how things turn out, and whether you send a card, flowers or food or whatever, the most important thing of all is: keep, keep, keep on sending messages of support via any means possible (mobile is good as people don't have to reply, in which case just leave an 'i'm thinking of you' message on the answerphone), and carry on doing it, again and again, even if you get no reply. Better say someting, even if it's clumsy, than withdraw for fear of offending her. I'm only just starting to be able to face replying to messages, 7 weeks down the line. But they are what have kept me going through the darkest days of my life. At my lowest points I've been reminded of that massive outpouring of love and support, and though I'm not a particularly spiritual kind of person, I think we have - including my little son - drawn from that the strength that we need to carry on.
Good luck to your cousin. I'll be willing her daughter on.