My understanding of the opinion is that it voided that provision of VAWA because there was no substantial nexus to interstate commerce (required by the Constitution in order for the federal government to regulate something). While, in my opinion, the decision is correct, there may be a debate as to whether violence against women affects interstate commerce.

Note that this does not mean that she cannot sue her just means that she has to do it under state law.

Incidentally, you cannot say that O'Connor, Scalia, Thomas and Rehnquist always vote the same way, or for that matter always get it right. While they are certainly the most conservative, they regularly disagree with one another, and their decisions are not always consistent. Except for Scalia--who is consistently a little frightening (though often correct).