Final answer:
Valentine considered Milly Erne to be her sister, indicating a deep familial bond. The term 'sister' is also used in different contexts to imply spiritual or emotional connections in other characters' relationships throughout the text.
Step-by-step explanation:
Valentine considered her "sister" to be Milly Erne. This is evident from the provided text snippets, where the term 'sister' variably applies to literal, spiritual, or intimate familial-like relationships. The references range from Sister Sara, an apparent figure of kindness and grace, to Jane Withersteen's acknowledgement of Bess as a girl or woman likely significant in Venters' life, and to the dramatic tensions in a passage suggesting familial strife and guilt by kinship.
The emotional and thematic resonance of the term 'sister' conveys both a biological relation as well as a deeper, possibly spiritual or emotional bond, as in the snippet mentioning Milly Erne, who is likely the person Valentine would have called her sister most earnestly.