My Summer of Love
(2004)

Mona (Nathalie Press) and Tasmin (Emily Blunt) plan of running away together but Mona's brother, Phil (Paddy Considine) intercedes and locks Mona in her room to pray for her.  Phil abandons his born-again Christianity when Mona fakes suicide and he hits her for it.  Mona runs away to join Tasmin to find out she has lied about her sister's death and she is returning back to boarding school.
 
Mona goes to their place in the woods where Tasmin later joins her and tries to explain her lies.  Mona attempts drowning Tasmin in the water since they had promised that they would kill one another if either ever tried to leave, but she stops and leaves Tasmin in the water. 

Thanks VK!