Why did this happen to Lucas?

Question

I just watched the Goodnight Mommy and one thing is getting me:

In Goodnight Mommy (2015) On a hot summer day, twin brothers Elias and Lukas (Elias and Lukas Schwarz) welcome their mother (Susanne Wuest) back to their secluded countryside home from what is implied to be a long stint at the hospital, where she underwent some serious plastic surgery.

As the days pass and the mother's behaviour becomes increasingly irritable and erratic, the boys start to question whether it is really their mum who has returned to them. Eventually the boys decide to take matters into their own hands.

This leads to the pair ultimately tying her up and basically torturing her in order to get some answers. In the final moments of the movie it is revealed that Lukas has actually been dead the entire time (he died in an unspecified accident) and Elias is riding delusionally solo.

So, the question becomes, how/when did Lukas die, exactly? The mother explains that it wasn't Elias' fault that his brother died. Why would she feel the need to say this? Did Elias in fact play a (probably accidental) role in his brother's death?

Answer 1

It is not clear from the film how Lukas died, however, it did happen at some point in the past (before the film's timeline) and we can assume that it probably happened while the boys were playing together (maybe the first scene when the boys are going down a cave is a hint). The divorce of the parents followed the accident which, we can assume, put Elias under more stress. The delusions also had started prior to the film's timeline as the mother was familiar with her son's delusions and she had played "the game" of caring for two before.

Answer 2

The scene in the beginning is the clue. The two boys are playing at the lake, counting. Looks like they're playing "who can hold their breath longer". Suddenly it's quiet. And we see the surface of the lake form bubbles. Elias calls out to Lukas and there is no response. It appears that Lukas might have drowned. This appears to be the guilt that's buried deep within Elias' mind. That he let his brother drown. This is the accident.