Heh, I'm a Christian and am not buying it. I haven't even read the book but got the readers digest version from my sister. I guess maybe what I should be asking is are there any rebuttals out there against the book?
Maybe just tell your sister that there are lots of books out there trying to discredit the Bible....don't believe them.
Then tell her if she really wants to know the truth herself, start reading the Bible and you GUARANTEE that God will reveal His true words, if she is truly sincere.
Forgot to add....
It is really easier to let God show someone the truth instead of trying to refute all of those crazy books.
Someone who is really sincere about finding the truth will find it.
To those who just want to argue about it, the truth will remain hidden.
So that's why I suggested to point her to the Bible instead of trying to refute every false doctrine that comes along.
There are plenty of rebuttals to the Da Vinci Code out there. Off the top of my head, I can think of one by Sandra Miesel and another by Amy Welborn. Miesel had an article published in Crisis magazine, which I think is still available on their website.
Cracking the Da Vinci Code... (Garlow & Jones)
Breaking the Da Vinci Code (Bock)
The Truth Behind the Da Vinci Code(Abanes)
And more...
All are at www.christianbook.com & other such places