I know this isn't a cute idea or even that exciting. But my big made me a print out tree (she made it on the computer) with all of our family on it and laminated it. So, I have a family tree I can hang up in my room.
Also, for my little, I did the same thing, and I put the family tree on a regular paper, and put that, along with my Big Sister Pledge and some pictures in it to start her album. She loved it!
You could also make a tree and put it on a plaque.. you can get premade wooden ones (more like award plaques) at craft stores.
Last one, I promise.

One girl I know, made a shirt with her little's family tree. But.. on the back where the tree was, she made mini letters of every girls first set of letters. So, her little had a replica of everyone's first set of letters.
Ann Marie