Perhaps you didn't see the show or the transcript. When asked if she thought it was "possible" that sexism played a part, she acknowledged that it is "possible." That is not an accusation. It is an acknowledgement of a "possibility." She did not say it was probable.
Should a First Lady, in that instance, reply that it is not possible? What if she were then proven wrong by some knucklehead who said he opposed Miers because she was a woman? Mrs. Bush can't "win" in this matter now, can she?
The better answer would be say something like:
"Oh, I don't think so, we have these disagreements from tiime to time and it's good for our democracy. But I don't think it's appropriate to get into name calling."
Couching something as "possible" doesn't really take the sting out of it as far as I am concerned. And this has been the White House line for several days now.