I bought my best friend a really nice journal once.
On the first page, I began for her what I told her was her "happy thoughts only" journal.
I wrote things about her that I liked and I also wrote things that I remembered about our friendship.
Then...when I gave it to her, I told her that each night before going to sleep, she had to write things that had made her smile throughout her day.
I am sure this can be a very nice sentimental gift for a sister if you have not done this already.
I told my friend that she was to read through her book when she was having a rough day (she is going through divorce)
I think, from the look on her face, that this gift was very sentimental indeed.
All these other ideas are great too.....Good Luck!