Possible meanings of this idiom are 1) "My heart now belongs completely to you" or 2) "I strongly desire to make love to you" (See: [[rc://en/ta/man/jit/figs-idiom]])
This is an idiom of affection. They are not actually brother and sister. Alternate translation: "my dear" or "my darling" (See: [[rc://en/ta/man/jit/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 4:8](./08.md).