Drafting/Planning Film Idea

Characters; Mother, Father, Two little girls.
Mother is your stereotypical woman that loves her children, cooks, cleans and works.
Father is an abusive alcoholic - abusive to both the mother and the children.
One girl is 10 and the other is 8; both are scared of their father and love their mother dearly.

Plot/Storyline; The father is an abusive alcoholic that always takes out his anger on either the Mother or the children. One day the father comes home and starts being abusive to which the Mother would normally not react to, however this time she reacts and end up murdering the father. Following this, the father becomes demonic and haunts the family by doing things such as possessing the children, making noises in the house, standing there as a black figure watching them etc. Towards the end the Mother tries to talk to the father and show understandng for why he was the way he was and for what he is doing now, she finally gets through to him and he lets go and leaves them alone.
Setting; The main setting will be inside a house (my house), however I have thought of ideas such as the little girls being in the park and seeing a black figure; therefore a park with a field may be another place I film my trailer.
Sound/Music; I'd include the usual 'horror' type sounds, however this is something I need to research more into.
Certificate; My film will more than likely be a 15 as I don't think it will feature many '18' elements. I will be researching into what I can include in a 15 trailer.