5

The phrase ”my sister, my bride“ is repeated many times in the book. Chapter 5 isn’t its first appearance by a long shot. I’m sure “sister“ is being used in a figurative sense because marrying your sister would be against the law.

You shall not uncover the nakedness of your sister, your father's daughter or your mother's daughter, whether brought up in the family or in another home. Leviticus 18:9

My guess at the reason for the phrase (which I think a lot of people recoil at) is that the husband is trying to convey the companionship and friendship in the relationship as well as the sexual attraction. It’s a phrase I think is meant to get across that there’s lots of aspects to the relationship.