If your sister simply deleted the partition on the disk that's relatively easy to recover from (though if you don't already have a good partition-management program the only free ones immediately available may be Linux-based). If she deleted individual files while running the system (you might want to password-protect it in the future) they may still be mostly recoverable if you install another system on your new drive to work from (do not do anything that would write anything to the drive that you want to recover).
http://www.techsupportalert.com/best-fr ... tility.htm is one place you can learn a bit about recovery options.