I somehow managed to get to the end, which was both abrupt and nonsensical. The brother has just been going from safehouse to safehouse, just barely missing his brother's arrival, then at the end the brother writes about a blinding white light and how he can't find anyone (implying that he's dead?). If that's true, there wouldn't even have been a way for him to write the letter in the first place. Furthermore, the purple gas looked like it stayed at about foot level, so everyone could have easily moved to the roof of a building or something.
I wished the checkpoints would have disappeared whenever they were visited so that the direction arrows didn't get confused.
Nice music and animation though.