Horrible dream

I had this horrific dream the other night. There was this large older two story house where I was visiting my sister (who died in August from Cancer of brain and lungs)and was accompanied by my son. There was a man that came to visit and Con and I left to run an errand. When we came back, the man was gone, Linda was dead and There was a video up on her PC. He had a digicam on and recorded himself raping and beating her to death. My son clicked it on and started to watch the murder. I shut it off and needed to leave to get help but was panicked because I wanted to take the harddrive with me so that I would have the proof of who did it. I was afraid if I stayed, he would return and murder myself and son, and if we left without the proof, he would never be caught. The dream is bad, but nothing I couldn't normally get over. The image of the murder of my weak sister and my son watching that is hanging with me. I wish I knew what my head was trying to tell me.