Hi M.,
What a sad loss to have happen in your family. This must be hard for all of you. You are a very thoughtful niece to want to do something for your aunt and uncle.
One thought that comes to mind is to do a scrapbook. You could gather some pictures together and then talk to various family members and have them write down memories of your cousin. Type up the memories in different fonts and put them in the scrapbook next to the photos. This is a great way to involved thw whole family, cousins, grandparents, nieces, nephews, aunts, uncles, etc. You could even include friends if you wanted, depending on how involved you want it to be.
Did the mortuary handling her service (if there was a mortuary involved) publish her obituary online? If so, was there an option for people to go online and leave messages for the family? You could include those messages in the scrapbook too. (Sometimes the newspaper has an online version that also published the obit and has an option for people to leave messages).
You could even leave some of the pages blank so your aunt and uncle could add some things of their own if they want too.
I'm in the process of doing something similar for a good friend whose dad died in hospice this summer, and I'll give the scrapbook to her for Christmas; some of the initial pain will have worn off, and she'll have the pleasant memories to look back on.
Anyway, I am so sorry for your loss. Your aunt and uncle are lucky to have a niece as special as you.
Blessings,
S.