That's wonderful. I hope that when my daughter discovers the truth, she will not ruin the magic for her little brother. And I hope she is as amazed as your son.
I have two non-believers and two believers. My ten year old daughter was just thrilled to make "reindeer food" (which she decided to do by herself), put out the cookies, and buy her first Christmas presents with her own money.
Both of them have been very good about being Santa's helpers and not destroying their sisters' illusions. I hope your daughter feels the same way. :)