Clever but disappointing. (Spoilers)
The twist was not believable at all.
If she wanted to cover up what her son did, they could have just said she jumped or slipped off the cliff.....
When I realised the detective was the one behind his daughters murder, I was hoping the reason for this would be more disturbing and fucked up than it was...
Turns out she was just covering up an accident for her son. Boooo!
That isn't very effective at all.
This should have been a horror film IMO.