Song of Solomon 4:9 (b) In this verse, as well as in the rest of this beautiful, poetical book, this word is used to represent the church. It is a type of the church. Many scholars disagree on the meaning of the various expressions used in these chapters, but it is plainly a description of a love affair between the church and her Lord.

Jeremiah 3:10 (a) The two nations, Judah and Israel, are represented in this and in other portions as being sisters of each other, as they really were (See Ezekiel 16:46).

Ezekiel 23:4 (a) Again the two nations of Israel and Samaria are represented as sisters, and they receive new names which describe their character.

Matthew 13:56 These are really the sisters of the Lord JESUS, although the Roman church denies the plain statement of Scripture and teaches that these were really nieces, and not sisters. We do not know how many there were. The statement uses the plural word, which means more than one. Mary gave birth to at least six other children besides JESUS. She was only a virgin in the birth of the Saviour. After that she was a normal mother with her husband Joseph.

Bibliography Information
Watson, Walter. Entry for 'Sister'. Wilson's Dictoinary of Bible Types. https://www.studylight.org/dictionaries/eng/wdt/s/sister.html. 1957.

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Wilson's Dictionary of Bible Types

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Sister