Who is Nerea in the novel, Hades, Argentina?

Nerea was Isbael’s younger sister. Nerea was also a member of Montoneros, but she was caught, arrested, and likely killed. Nerea was pregnant when she was arrested. When Tomás returns to Argentina to visit, he meets Vivi, a girl who is supposedly Nerea's daughter. However, Tomás later learns that Vivi was likely a different orphaned baby given to the family by the military after Nerea's death.