Why does Pollyanna live with her aunt in the story?

Pollyanna is sent to live with her aunt (Polly Harrington) after the death of her father. Her mother had died previously.

From the text:

‘Dear Madam:—I regret to inform you that the Rev. John Whittier died two weeks ago, leaving one child, a girl eleven years old. He left practically nothing else save a few books; for, as you doubtless know, he was the pastor of this small mission church, and had a very meagre salary.

‘I believe he was your deceased sister’s husband, but he gave me to understand the families were not on the best of terms. He thought, however, that for your sister’s sake you might wish to take the child and bring her up among her own people in the East. Hence I am writing to you.