Ben is a jerk

As far as older brothers can go it's not that far off. Not to mention Ben blames her for the death of their mother. The entire movie is about coping and growing because of it instead of being bitter.

what part of "ten-year-old boy" don't you understand?

It's a million less worse than the parents of a certain overpraised movie. And it's needed to push the plot, as the anger of losing your beloved, smart and constructive maternal mother.

He's not s jerk, he's a child struggling to come to terms with the trauma of losing his mother.
In his unconscious his mother was taken away and replaced by a baby.
He loves her deep down (as becomes clear when he risks his life several times to save her); but he doesn't know where else to put his anger.
He also has a lot of jealousy as she appears to him to take his fathers love- quite a normal sibling reaction, it's called 'sibling rivalry'.

So do I, but unfortunately it's the way I treated my younger sister when I was growing up, and the way my eldest son treated his youngest sister.

--------
My top 250: http://www.flickchart.com/Charts.aspx?user=SlackerInc&perpage=250

He showed his own disapproval for the way he had treated Saoirse.

there's a highway that is curling up like smoke above her shoulder

Many (most?) siblings are jerks to each other while growing up. Part of growing up is learning how to treat the people around you with love and respect, so it follows that people who are still in the process of growing up may not have learned that yet.

Ben does improve quite a bit in that regard, over the course of the film.