The stranger, Victor Frankenstein, says he was born in Naples and grew up in Geneva, Switzerland. His father, Alphonse, and his mother, Caroline, first became close when Alphonse's friend and Caroline's father, Beaufort, died. Alphonse became Caroline's protector, and eventually married her.
Victor's childhood is innocent and perfect. His family life is perfect domestic bliss.
When he was five, his mother discovered a beautiful blond orphan girl named Elizabeth Lavenza in an Italian village and adopted her.
In the 1818 edition of Frankenstein, Elizabeth is Alphonse's niece (and Victor's cousin).
Victor, his parents, and all the Frankensteins adored Elizabeth. She became to him a "more than sister." The two children referred to each other as cousins, rather than brother and sister.
More domestic affection, and the relationship between Victor and Elizabeth hints at future romantic love.