How to recover my files on a Mac OSX after overwriting..?

Hi I did a little cleaning up on my hard drive today by moving some old files to my external hard drive. I moved my old photos to a "PHOTO" folder to my hard drive and deleted the files right after from my MAC to free up some space. After I was done, my sister accidentally overwrite that folder from her own MAC with the same name - PHOTO - but it was empty. As a result, all of my photos are gone! Is there any way to recover them? O.o