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They consulted their village priest, and the result was that Elizabeth Lavenza became the inmate of my parents' house--my more than sister--the beautiful and adored companion of all my occupations and my pleasures. (1.6)
Literally the only reason that Victor's family adopts Elizabeth is that she's pretty. Oh, sure, she's charming—but being charming seems like a subset of being pretty, not something separate.
Everyone loved Elizabeth. The passionate and almost reverential attachment with which all regarded her became, while I shared it, my pride and my delight. On the evening previous to her being brought to my home, my mother had said playfully, "I have a pretty present for my Victor--tomorrow he shall have it." And when, on the morrow, she presented Elizabeth to me as her promised gift, I, with childish seriousness, interpreted her words literally and looked upon Elizabeth as mine--mine to protect, love, and cherish. All praises bestowed on her I received as made to a possession of my own. We called each other familiarly by the name of cousin. No word, no expression could body forth the kind of relation in which she stood to me--my more than sister, since till death she was to be mine only. (1.7)
Victor sees that Elizabeth’s beauty is the reason people love her. Yet this seems to be the reason he loves her himself.
When my father returned from Milan, he found playing with me in the hall of our villa a child fairer than pictured cherub—a creature who seemed to shed radiance from her looks and whose form and motions were lighter than the chamois of the hills […] the result was that Elizabeth Lavenza became the inmate of my parents' house—my more than sister—the beautiful and adored companion of all my occupations and my pleasures. (1.6)
Okay, seriously: aside from the fact that it's totally creepy that Victor's parents essentially adopt a child for Victor to marry, what are the odds that you would even want to marry someone you'd grown up with? Like, grown up in the same house with? There's a reason (besides the obvious genetic ones) that we don't marry our siblings.