And we see her spirit/ghost walk past the oblivious firefighters in the penultimate scene.

You're completely ignoring the fact the priest talks with the mother when he takes Elias back home, but no one EVER talks or interacts with Lukas. So no, the mother is alive and Lukas is the one who died.

Lolno.
Fine, fine, I'll leave! But first I'm going to bother these peanuts! Hmm? Yes? Hmm? HMM?

The twist is - everyone assumes, like the movie "The Other" - that
Lukas is dead - but in reality the boys are alive, and the twist is that the Mother is dead, and she's in the cave. Elias can't accept that she's dead, and fantasizes that she's alive and Lukas tries to get him to see the light. The boys successfully keep her death a secret until the fire. (See the delivery man stocking the freezer, and the Red Cross visit) - at the end, after the fire, both boys are taken into shelter.

Harsh summary but I agree. Ultimately it was disappointing. But few scenes were worth the trouble for myself.