A masterpiece...almost.
This was an extremely moving story. I loved how the store clerk was able to feel empathy for the boy, although it was a little complex. He felt empathy because he wasn't able to make his sister happy (obviously, because she died) and because, after that point, he didn't even /have/ a sister. In any case, I could see the empathy and it was definitely shown well.
The problem is, it's missing something. The illustration was fine, but the animation was very robot-like. It's not like it wasn't smooth or anything, but it was like the characters' limbs were one-dimensional; they didn't have depth at all and the actual animating of their limbs was very mediocre. I was not impressed with that.
Another problem is, the mother and sister were hit by a truck? Just like the store clerk? That's a little unimaginative, although I can see where you would think it would be memorable since the clerk read the paper and his own history would make him feel even more empathetic, but it was simply not a creative twist. Maybe you should have replaced the truck incident with something else more relevant to the boy, but I can definitely see where you were going with it.
All in all, this was a memorable and emotional flash; I love seeing flashes like this executed so well. However, the story and animation aren't perfect. I'll give you an 8/10 for this as you obviously paid close attention both the story and the animation, but simply did not perfect them.
Good work.