Since David was married to Michael, Jonathan was his brother-in-law (1 Samuel 18:27). In this case, however, he was remembering the covenant which they made with each other (1 Samuel 18:1–4; 1 Samuel 20:12–17; see further 1 Kings 9:12–13; 1 Kings 20:32–33).1
26 I am distressed for you, my brother Jonathan; very pleasant have you been to me; your love to me was extraordinary, surpassing the love of women.