Sounds very sweet what you are looking to do. I was actually reading some of the replies. I had a thought. If you could take a picture or several pictures during the process. Have the pictures matted. Of course, do this before the pictures are in the frame.
On the matte you could use glue to make some scrolls or meaningful words and put the sand down. Do it all around the matte....just the top or the bottom. You might have to use stensils. Try a test run first.....just an idea....not sure how it would work! :)
Just my opinion - but I think three pictures. One before the house, once in mid-construction, and the finished house. Would certainly be a daily reminder of your love and thoughtfulness of your sister. And nice reminder to see how far your sis and her hubby have come.
Good Luck.
K.