Lenora

From Wikipedia:

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)

She's not his half-sister or "adopted step-sister".

Grandmother Emma tells Lenora and Arvin they are step-siblings to make sure the two children treat each other like brother and sister, which fosters a real family relationship and helps avoid sexual/romantic inclinations when the children mature.

The kids and the grandmother were all the family they had left between them, the grandmother wanted the kids to feel family love, not child abandonment pain. They all suffered great generational family loss.

Lenora's mother Helen was a neighbour to Emma. Helen's family's house burned down, killing Helen's family. Emma took Helen in. She wanted Helen to marry her son Willard. Both young adults married different people (Helen married Roy and had Lenora, Willard married Charlotte and had Arvin), and all four young adults exited life early, leaving the two kids behind, raised by Grandma Emma.