They are siblings. When they were younger Sonia was being targeted by a Mafia leader who liked... younger girls per say. The Komaedas were business partners with the King and Queen of Novoselic(Is that what it's called?), the king and queen wanted a daughter but the queen fell ill and couldn't safely produce children. They struck a deal, The King and Queen will become Sonia's parents and will protect her. However, the Komaedas were to never see her again.
Nagito actually remembers all this.
Sonia does not.
Nagito wants to keep it that way, even if it means to never have a family again. Besides, who would wanna find out that they're related to trash like him.
