Individual Idea

2010:
An 11 year old girl (Alice) runs into an abandoned house after her dog escaped his collar- ahead of her parents. She explores and finds the dog sniffing around the body of a dead teenage girl in a basement of an abandoned house. The dead girl is the little girl's missing sister, Lilith that disappeared 9 years ago neither she or her abductor was found- until now.

2017:
With the case still open Alice tries to find answers about her sister and notices a figure following her
after school daily and that he appears to be following her home. She traces her sisters last known day from her home to a corner shop and see's the figure at the end of the road taking photos of her. She chases him into a warehouse where she loses sight of him- she goes into an office type room and see's  the walls covered in photos of her mixed in with ones of her sister which have red 'X's' through them. Something drops to the floor behind her and she spins round and gets knocked out by the figure with a pole. She wakes up tied to a bed with a selection of knives next to her and the figure selecting them carefully.
She asks who they are and what they want and the figure turns to reveal her sister- alive- who explains that their parents had tried to kill her as a cult sacrifice but she escaped and that now Alice is 18 they will do the same thing again; this time finishing the job to gain access to what they believe will bring them a higher power.
Lilith unties Alice and introduces her to Martin, Lilith's companion who is part of an organisation trying to take down the murderous cult her parents are part of.
Sending Alice back into the house as a form of bait is the only option and Martin and Lilith plan to rescue her and reveal the whole thing with the help of the Organisation named Phoenix. 
The ceremony goes ahead but as Martin and Lilith sneak in they are intercepted by Cult members and a fight ensues whereby Martin is beheaded and Lilith is stabbed in the side but struggles to help Alice.
As Lilith reaches the Alter, the clan leader is raising the knife above his head ready for the sacrifice and as the knife comes down Lilith pushes him aside, knocking over a candelabra in the process- setting the room on fire. Alice is able to go to Lilith but she dies as she gets there, leaving Alice to face her parents on her own.