EXOD281 We are obligated to honor our parents-in-law. Rabbi Naftoli Tzvi Berlin writes in Haamek Dovor that although Moshe's wife and sons were also coming to him, Moshe made a point of greeting his father-in-law first. He did so because of the obligation to honor one's father-in-law (Yorah Daiah 240:24). (The B'air Hatev adds that a person is also obligated to honor his mother-in-law.) Pele Yoatz (section chomov) writes that a son-in-law (or daughter-in-law) should appreciate the fact that his parents-in-law spent much time and effort in raising their child who is now his wife (or her husband). In gratitude, a person should always treat his in-laws with the proper respect.