You were lucky. In similar situation my friend noticed a tote bag I had just made (for her Christmas present) and said she loved it....could she perhaps have it? Well, I said yes, but not until Christmas. Then I told her I had made it for her all along.
But your sister will be surprised! She may not even believe it was right there and she didn't see it.
I don't think I would believe that if my sister told me that.
Dina