ending
I found the ending a bit anticlimatic. By the time it happens, you already know the brother is unhinged and the sister has to go along with it to save her child.
But what is not clear is whether they have been playing this game for their whole lives (including the past four years)? If so, the mother has been endangering her daughter by requiring her to play along with this sick game. If this was a new thing, she certainly figured how how to play quickly.
And what happens at the end? Does the brother get put into an asylum? It was odd that of all things the brother had done, the cop said "you shouldn't have lied about the name of the boat". Why did he lie-- to get caught?-and why is Olivier so caught up in that- he should have just arrested him and been done with it.