She's not like the rest of them. She's vain, and shallow, and she has no conviction in anything. But she believes in family. She believes in the accidental, involuntary bonds that form between those that are blood related, and she believes that those bonds triumph over everything else. She never forgets Sirius, who left, but who's still her cousin. She never forgets Andy, who married a Muggle, but who's still her sister.
She doesn't talk about them, because no one else will acknowledge them, but she never forgets.
And in that battle, the last battle, her family is at stake. And that doesn't sit right with her. And she knows, in her heart, that the only way her family can come out of this whole is if it's over. It all has to be over.
So she lies for a boy she knows only by name; she lies to save the boy who's been nothing but trouble, because she knows that he is the only way this can all end. She lies for her family.