sister

Definition:

A sister is a female person who shares at least one biological parent with another person.

In the New Testament, "sister" is also used figuratively to refer to a woman who is a fellow believer in Jesus Christ.
Sometimes the phrase, "brothers and sisters" is used to refer to all believers in Christ, both men and women.
In the Old Testament book, Song of Songs, "sister" refers to a lover or a spouse.

It is best to translate this term with the literal word that is used in the target language to refer to a natural or biological sister, unless this would give wrong meaning.
Other ways to translate this could include, "sister in Christ" or "spiritual sister" or "woman who believes in Jesus" or "fellow woman believer."
If possible, it is best to use a family term.
If the language has a feminine form for "believer," this may be a possible way to translate this term.
When referring to a lover or wife, this could be translated using a feminine form of "loved one" or "dear one."