The Mitzva of honoring parents extends to other relatives. The main difference between our parents and any other relatives is that toward our parents we have two obligations:
1. Honor and 2. Obedience/respect of their authority (mora). While toward other relatives is limited to honoring.
1. One should honor his older siblings. For example, it is customary that a younger brother or sister will stand up when the older brother goes ('ole) to the Torah, etc.
2. In our community grandchildren stand up for their grandparents when they go to the Torah. They would also kiss their grandparents hands when they come down from an 'alia, as a sign of love and honor, and after Friday night's Kiddush. Grandparents are also honored in special occasions, like being the Sandak in a Berit Mila, etc.
3. Honoring also applies to one's in-Laws. We learn this from David haMelekh who called King Shaul : 'My father' (Shmuel I, 24, 11. David was married to Shaul's daughter, Mikhal) that one should honor his father and mother in-Law as one honors his own parents. This rules applies even if one's father or mother in-Law are of a young age. The son/daughter in law should stand up for them, etc. In Sephardic communities it is also customary not calling in-Laws by their first name, but addressing them with the honor one addresses an important person. Or calling them 'father' / 'mother' as one does with his own parents.
(Yalkut Yosef, Kibbud Ab vaEm, bet. 496-515)
Rabbi Yosef Bitton. YMJC | 130 Steamboat Rd. | Great Neck | NY | 11024