So I just completed Limbo and I have no idea what the ending means (spoilers).

I haven't played it myself but can I recommend putting spoiler or something before the end part :)

1) USE. GOOGLE. IF. YOU. WANT. TO. KNOW. ANYHING.

2) yes, they both are dead. other information you can read by GOOGLING it.

hmm, maybe the dev want us to use our own imaginations about the story
i still like that game :D

I think the boy and his sister were killed in a car crash. That explains the exact moment where the ending starts: the boy flies through glass in slow motion. So, as the name of the game sugests, the ghost of the boy is trapped in a LIMBO where he is forced to do the same things over and over again, until his "goal" (to be reunited with his sister) is reached and his soul can finally rest.

They were playing in the tree house, something fucked up and boy fell down, he died.
Then you make your way to that glass, you fly through, see your sister and she's burying you. End. Period.

Here.

There isn't an official explanation, so there is room for individual interpretation. The only thing which is almost certain is that the boy is dead. Beyond that there are possibilities. Google may offer some answers. Here are some theories that I remember:

• They are both dead and just wandering around in the Limbo (forever?)
• The whole game happens in a fraction of a second in the boy's mind. Like, right before you die your whole life flashes before your eyes. Maybe it is an abstract variant of his life. The spider represents his fears, the people are the ones who bullied him, the industrial part could be a metaphor for his working life. Or maybe it is just random stuff. However, in the end, the accident happens and the boy is dead.
• Only the boy is dead and is looking for his sister. She, however, survived and he is damned to keep on looking for her.

the boy was in limbo (e.g. he died but he is not fully into heaven)
he got through the treacherous place of LIMBO, where his mind came up with it probably, and the game reflects things in his own life (spider was someone who bullied him or something probably)
finishing the game, he breaks through the line seperating LIMBO and heaven, and meets up with his late sister. wouldnt surprise me if they made a LIMBO 2 featuring the sister's playthrough

I think it's one of those games where you'd better not seek any clear explanations (because there may even not be one). Some things are ment to be percepted by feelings, not by mind. This game is like a dream, and you don't often try to rationally explain your dreams, do you?