What does mildred think is at the door? what is at the door?

ell you have to think about it all. in the flashback it shows anna (the main girl) leaving a party early cus that guy was tryin to hook up with her and she goes home and sees her dad having sex with the nurse. so she goes and gets a bucket of gas and looks like she is planning on killing the nurse somehow (im not really sure how, it does show her looking at a map of the house) then she leaves the gas faucet on by accident and her sister slams the door and the a candle falls over, then her moms bell rings and so alex (the sister) goes back in the outhouse to help, and then the gas hits the candle and blows up the outhouse. im not completely sure whether the dad and everyone knew that anna started the fire (i think they do know) but either way she cuts herself and the dad sends her to a psych ward and she cant remember anything from that night. as she is leaving the psych ward the last advice the doctor gives her is "finish what you started." she is by now, completely insane and imagines her sister still being alive. she imagines her step mom being some crazy killer who killed those little kids by drugging them and stabbing them. it is just stuck in her head, ever since he saw her having sex with her dad, that her step mom is a horrible person. im assuming that the shots the step mom gave anna were medicine to calm her or sedate her until she calmed down because everyone except anna knows that she is crazy. she then imagines the sister killed the step mom and finally comes to reality when her dad is screaming at her. she realizes she killed the step mom and then remembers what really happened that night. she remembers how the boyfriend actually saw her start the fire and when he tells her that he knows...she drops him off the cliff. the step mom didnt want the boyfriend coming around the house because she didnt want him reminding anna of what really happened that night. so when she killed the step-mom she "finished what she started." pretty confusing (especially the imagined sister) but im pretty sure thats it. as for mildred kemp, im pretty sure there was no real significance of her other than to just prove that the step-mom was actually completely innocent, which is sad because she was so hot. haha hopes this helps. i actually thought the movie was pretty good, definitely made you think