Call your parents. Tell them it’s an emergency. Tell them what’s happening. If your sister wasn’t aware of the real dangers of the house, neither you nor she are to blame for this- it’s on THEM. If need be, call 911, report a home intrusion. If you need an explanation, tell them this: “I was home and I heard someone walking around downstairs and assumed it was my sister and ignored it for a while. But she just messaged me that she’d left a while ago and accidentally left the door open. And I still hear someone walking around.”