What a neat way to honor your sister and those who cared for you!
Two groups you could consider:
The Preemie Project accepts a wide variety of donations and is run by some very nice people. They also have a lot of good information, size charts, and pattern links on their site.
Lucy Bug knit-along is for those who want to encourage each other while making bereavement gowns, hats, booties etc. for little ones who flew home too soon. Items can be sent to Lucy Bug for hospitals in Utah, or people can join in and share what they are making and send to any organization/hospital they prefer. While most of the items are for loss, anything people want to send for preemies in the NICU are also distributed for that purpose. Lucybug has a few patterns on their site with plans to add more. Lucybug is run by my daughter in honor of a friends niece.
Both sites have photos of things people have made and might give you ideas on what you would like to do.