Is there something I could have done to stop my sister's murder?

I am so sorry. There is nothing you could have done. You didn't have time to react to the situation in any way. Even if you could have reacted to the situation in time, there would've still been a chance that you couldn't save her. I know how hard it is to lose someone. The event must have been extremely traumatic for you. I would suggest seeing a counsellor or talking to someone you trust. Death is always a shock to the system.