Question: "Why does Solomon refer to his wife as his sister (Song of Solomon 4:9)?"
Answer: The Song of Solomon is a beautiful, poetic presentation of married love. Chapter 4 deals with the wedding night; as the bridegroom and his bride consummate the marriage, they speak to each other tender words of praise and affirmation. Four times, in speaking to his wife, the bridegroom calls her “my sister, my bride”—or “spouse” in some translations. “How delightful is your love, my sister, my bride!” (Song of Solomon 4:9; see also 4:10, 12, and 5:1).
Solomon had not married his actual sister, so we can dispense with that theory right away. Rather, the term sister was a common expression of closeness and love. In ancient Egyptian love songs, “my sister” was a customary name for a female lover. It was a term of endearment that emphasized the permanence of the relationship (a sister never stops being a sister). In giving his bride a double title, he shows her double honor: he loves her with the passion of a spouse and with the purity of a sibling. Blood is thicker than water, and the bridegroom wants their relationship to showcase the permanence of a blood relation.
There is an interesting parallel in Song of Solomon 8:1, where the bride says, “If only you were to me like a brother, / who was nursed at my mother’s breasts! / Then, if I found you outside, / I would kiss you, / and no one would despise me.” Here, the bride yearns for the freedom to express her love, even in public. Outward shows of affection were taboo in their society—with an exception made for siblings. So, the bride wishes for the freedom to kiss her husband—the same freedom a sister had in kissing her brother in the public square.
We still use the term sister today, in a different context. True, a husband might not call his wife “sister,” but a group of women may express the closeness of their relationship to each other by forming a “sisterhood” and referring to themselves as “sisters,” even though they are not blood relations.