José *spoiler*
So, the mother was the murderer.
Last time shown, José was very sad because the dog was missing.
Then the mother comes as a traditional bad dressed old spanish woman yelling to hell that José was missing. Simblings run outside and are attacked by her.
I'm sure José was killed ever before the mother warned them. He was never seen or heard again.
Was he the burned corpse? But that corpse was too old to be him.