A childhood belief about father christmas

I have two older sisters, and when we were little, my middle sister and I wrote letters to Santa Claus. Showed us where we could send them and get letters back(Of course, places in HER room). My oldest sister pretended to write letters to Santa like us, but really she was writing replies to the letters we had already written. She would put all of our letters in the sending spot and make sure we watched. Then while we waited outside for replies, she made excuses to go in and switched her letters to the recieving spot and hide ours somewhere where we wouldn't find them. She would come back out, and a few minutes later she would say, "Let's go see if he wrote back!" We would check, and of course our letters were gone and "Santa's" letters were there. I don't know when my sister caught on, but I didn't realize what she had done until years later!