Definitely what Giraffe_Monster said!!
Elias feels responsible for some tragedy that killed his twin, so his messed-up mind believes him still to be alive. Mother used to play along with this, but is now separated from her husband, getting into online dating, and obsessing over her looks vs her surviving son's mental well-being. He gets the idea in his head that her new detachment indicates that she's an imposter and does some nasty crap to her, culminating in her burning to death as he runs upstairs and meets the same fate. We see Mom's ghost walk unseen past the firefighters, on her way to pose happily in death with her sons for the closing shot. The End.

Lol I don't know what the f_ck the person above is on about, but the Mother was the real mother all along. The kid was just messed up.
Fine, fine, I'll leave! But first I'm going to bother these peanuts! Hmm? Yes? Hmm? HMM?

thank you

Spoiler Alert
The twist is - everyone assumes, like the movie "The Other" - that
Lukas is dead - the boys are alive, but 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 keep her death a secret until the fire. (see the delivery man stocking the freezer, and the Red Cross visit) - at the end, both boys are taken into shelter.