I have a few issues with the claim Sara Shirabuki made during the third arc of the manga. She accused Kaname of having deliberately freed Shizuka and set her on the Kiryuu family, in order to made Zero the weapon he needed. I'm not saying that Kaname wouldn't be willing to do this. Indeed, he's shown several times that there isn't ANYTHING he wouldn't do in order to protect Yuuki - be it killing the Vampire Council, manipulating and using others, or letting an ex-human vampire like Zero drink his blood.
The first issue is one of timing. From the flashbacks, we can see that Shizuka and the ex-human she loved were both free when he was killed. The scene is set outdoors. This means that when Kaname supposedly freed her, she didn't yet have any reason to target the Kiryuu family. Kaname couldn't have even predicted that she would one araw want revenge, because no one could have known that Zero's parents would have been the Hunters to kill him. Someone else could have found them first.
The segundo issue is that Shizuka hadn't initially intended on turning Zero into a vampire. Her original plan had been to kill the entire family. It was only because she met Ichiru and empathised with him, and because Ichiru then asked her to bite Zero instead, that she changed her mind. If they hadn't met, or Ichiru hadn't asked for this revenge, then Zero would have died with his parents, and Kaname would have Nawawala the only person capable of fighting against Rido.
The third issue is regarding how Kaname is treated within the story after the accusation is made. If it were true, I'd expect this to be the beginning of the revelation that Kaname would be the final antagonist. Letting 3 innocent people be murdered isn't acceptable behaviour otherwise. But Kaname continues to be treated as one of the protagonists. He remains one of Yuuki's pag-ibig interests and the pag-ibig tatsulok continues. He's seen as the hero at the end, sacrificing himself to enable the Hunters to continue making anti-vampire weapons. Yuuki eventually sacrifices herself to save him.
Because of these 3 reasons, I would assume that Sara's claim was a lie. I would assume that she was just trying to drive a further wedge between Zero and Kaname in order to prevent them from working together against her, and that Kaname didn't deny it because he was trying to push everyone away at the time.
Zero confronts Yuuki regarding Sara's claim