Monsters and demons

Sabrina has lived in the Schmidts' big house for as long as she can remember and is the employee since after her mother's death. She is in charge of feeding the family, cleaning the mansion and a task that they don't dare to do, although it's vital: traveling to the dimension in which the vital watershed tree is, a fruit that they must eat at least once time in the week to stay alive because they, far from being human, are monsters.
After years of doing the same, Sabrina finds the oldest son of the family dead. Despite being a major death, the family doesn't seem to care about it and even hides it, which arouses suspicion in Sabrina. In search of knowing the truth, Sabrina finds out that the family hides more things than she could imagine and that the death of the oldest son was not random.
Who are they really?

This work is mine. The original is "Monstruos y demonios" in spanish, on my spanish account.