The Devil All the Time
How is Lenora step-sister to Arvin in movie "The Devil all the Time"?
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In 1957, Charlotte is diagnosed with cancer. Willard believes he can influence God with fervent prayer to remove cancer from his wife's body. He prays to God and sacrifices Arvin's dog, kneeling before a rustic cross he had erected in the woods behind his house. Nevertheless, Charlotte dies and Willard commits suicide. Arvin, now orphaned, goes to live with his grandmother Emma, where he meets Lenora, who becomes his adopted "stepsister".
You can read to entire plot summary here: https://en.wikipedia.org/wiki/The_Devil_All_the_Time_(film)#Plot