So, my thoughts were that It wasn´t the girl buy her family that died. Difficult to explain exactly right now, but seemed to me like the girl was the one asking for the mediums help, not the other way around. And she stayed in the house after the rest of her family left.
shareI think that the idea was that she was seeing lots of glimpses of the future. Not only the horrible vision of her own body, but also premonitions about what would happen, including what she tells the medium about her mother leaving the room.
She did seek the medium's help because she wanted to understand what was happening to her. She didn't know how to process what she was seeing.
Essentially she was being haunted by her future self, and then once she has died her family is also "haunted". Once she was dead (truly dead, not just as a future echo), she wanted her family to understand why she had been so withdrawn and depressed.
It wouldn't make sense if the family died, because the movie is shot in documentary style and both the family and other secondary characters (police, neighbors) talk about the girl's death and the aftermath for the family.