Group idea for plot synopsis

A family move house, to a remote country area, however the mum and dad are called away on an urgent family issue, leaving the children, a boy of 18 and a girl of 17 to look after the house. After a while the pair start to get bored and decide to explore the new area, when they come across a seemingly abandoned house and take a look inside. Whilst inside, the wind blows, slamming the door shut and startling the girl, who then kncks over an expensive looking urn. Ashes fall out over the floor. They hear a loud bang and a creaking door from upstairs, they both look to each other and run out laughing and teasing each other over how scared they were.
They return home and carry on as normal, only the girl starts to complain about feeling uneasy, as if someone else was around, her brother dismisses this, saying she is being stupid and telling her to calm down. Her feelings start to get worse, but the brother still doesnt believe her and begins to get angry with her for "being so stupid".
One night, the girl wakes up in the middle of the night screaming, claiming something was watching her over her bed, her brother is very annoyed by this, making her out to be a liar and causing drama, possibly trying to scare him. They get into a fight and he takes to ignoring her, she begins to feel very alone. He heads to the shower to relax and on coming out, notices a message in the steam on the mirror, thinking it was his sister playing tricks, he looks around to find her but she is sat outside crying. He grabs her and tells her what has happened and that he now believes her. They run into the house and try to contact their parents but find that the phone line has been cut, they hear what sounds like heavy breathing and run upstairs. As night falls they decide to sleep in the same room to keep each other company just in case anything happens.
The next morning, the girl wakes up and turns to her brother finding him dead and her top splattered in his blood, she catches the heavy breathing sound again, thinking the killer is still around, she forces herself to stop screaming and runs to hide in the closet, shaking and peering through the keyhole. Realising that there is no contact to the outside world and that she has no other way out, she makes a break for it. Only, whilst running, she passes a mirror, catching a glimpse of someone she doesnt recognise. She goes back to take a closer look and sees a haggard old woman. She screams and runs out of the front door, right into her parents arms, who after being worried by the telephone line not working, return home early. At this point the girl is still covered in blood and rambling on about being possessed. The parents find the body of her brother but yet can get no information out of her and so, after much grieving for the happenings, take her to a mental institute.