You know what they say when you have a near death experience; They say that your life flashes right through your eyes. And that is what this game is about. A brother and a sister were in an accident, but just before the brother dies, he recounts his whole life up till the point of the accident.
At first you are in a forest, as a child who has no obligation to the world, he could just wonder around. The spider I'd say is a metaphor to all the insecurities you have to go through in your childhood. Then as he got older, and maybe started school, he was bullied. Hence the other children at this point in the game. After school, he started working in some kind of a factory. The way I see it, the girl he sees the first time is not his sister, but maybe the love of his life, or maybe some kind of unrequited love since he never actually gets to her.
Now I'm not so sure about what is going on in his life after that point, but I'd say that when you find the switches that spin the world, his life was at a turning point. And maybe later on his life went so upside down that he actually committed a suicide. But the way how they died is clear: It was a car accident. In the main menu, if you look at the thing in the upper right corner, you'll notice the thing is actually in the shape of a car. The front wheels are visible. And right before the end the main character actually flies through a glass: It's actually the windshield.
So that's it. This game is a about a car accident and the last moments before the guy's death.
It makes sense up until the suicide part. I would change that turning upside down wasn't suicide. Just the hardships that a lot of people go through that they feel turns their world in circles.
Also, the game takes place as a little boy because that is when we feel most vulnerable. We feel small and defenseless even though as children we feel we can figure out the world. He might have just been remembering just how little he knew as he took his last stroll through his life.