How I Live Now (2013)

I don't really know where to start with this film.
Daisy (she refuses to answer to her given name Elizabeth) portrayed by Saoirse Ronan is an American teenager who is sent to England to live with her aunt (Anna Chancellor, who has no more than two minutes of screen time) and her crazy cousins who sort of live off the grid.
Naturally, she is angry at first and miserable but comes around rather quickly, thanks largely to her attraction to the eldest cousin, Eddie (George Mackay). Despite a nasty bombing in London, they are very happy and fall in love only to have everything destroyed when the soldiers come. The girls are taken to a house with militia guarding everything and work on the farm dealing with vegetables all the time while the boys are taken elsewhere.
The rest of the film is Daisy and little Piper (Harley Bird) walking back home. They manage to escape fairly easily and walk through the woods. Sure, they do run into trouble and Daisy kills two men who try to kidnap Piper and she finds Isaac's (Tom Holland's) dead body but not Eddie's. However, my main problem with this is that she forgets the water tablets (terrorists have poisoned the water sources) so they walk for days without a source of reliable, safe drinking water. So that's a problem.
But they get home and even find a battered and bruised Eddie, dealing with shell-shock. His burns and bruises heal but he doesn't utter another word for the rest of the film. But Daisy still loves him and swears (via a long-awaited voice over) that she will wait for him forever. Oh yeah, the war is also over, just as suddenly as it began.
So, yeah, some big problems with this film. The acting is mostly solid though the scene is where Daisy foolishly burns her ticket out, Eddie needed a better reaction. However, that was only a minor problem. There were two big gaps. First of all, why is the world so troubled, we don't really know and it is never explained. And why does Daisy hear bizarre voices in her head? Does she have a mental disorder or has the government planted something inside her head?
It is a shame. Sure, this was based on a book, but the film should stand by itself and this one doesn't. Grade: B-
Side Notes:
-Kissing and having sex with your cousin is beyond disgusting. I've said it before and I'll say it every time it happens.
-They do a good job of making Ronan look more grown up by the end of film by simply growing her bangs out.
-Why does the host mother serve the same food each night?
-Can Eddie talk to animals? If so, that's another major plot hole.