For what mysterious reason has Helio, 15 years old, placed with the Dainville family after the accident that left him mute?

After a head injury, Helio, a fifteen-year-old passionate about science, can barely communicate anymore. His cognitive abilities remain intact, but he is now mute.
With his mother also in shock since the accident, he is placed in foster care with the Dainville family, a choice she had made in the event of her demise. Mila and Ruben, the Dainville children, each welcome him in their own way, but for Bianca, the housekeeper, Helio's arrival resurrects distant memories. What secret has she been hiding from everyone for years?