This is an idiom of affection. They are not actually brother and sister. See how you translated this in [Song of Songs 4:9](./09.md). Alternate translation: "My dear" or "My darling" (See: [[rc://en/ta/man/translate/figs-idiom]])
This Hebrew word can refer to a woman who is married or to one whom a man has arranged to become his son's wife. If your language has a polite word that a man would use to his wife and that has not been used yet in this book, you could use it here. Otherwise you could use any polite term a man would use with his wife. See how you translated this in [Song of Songs 4:9](./09.md).
"is a garden that no one can enter." The garden is a metaphor for the woman, and the lock is a metaphor for her still being a virgin. (See: [[rc://en/ta/man/translate/figs-metaphor]])
"a spring with a cover on it." The spring or well is a metaphor for the woman, and the cover is a metaphor for her being a virgin. (See: [[rc://en/ta/man/translate/figs-metaphor]])