I would suggest having a talk with her,but, explaining the whole Spirit of giving, the real story of St Nick, (the real person) and the spirit of Santa . He is alive because that feeling is alive in 95% of us. It is just easier to show it at the Holidays but, we should all one up it to be giving all year long, even if it's just your time ,to volunteer somewhere,anywhere to help somebody else.
Explain to DD that she shouldn't want to one-up her brother in this case , as it would cause alot of unhappiness for him. When you give her the talk ,re: above, maybe you should include brother in that conversation. It really is more about the Spirit of giving than the Image of Santa. Santa is just easier for very young children to get . Best of Luck, (and I'm with you on keeping childhood dreams,images, etc, aound as long as possible for kids. They grow up way toooo fast. C. S.