Just as the Torah requires respecting one’s parents, so must one show respect to his parent’s spouse, even if the spouse is not his biological parent, as long as the parent is alive. The Gemara in Masechet Ketubot (103) infers this Halacha from the verse, "Kabed Et Abicha Ve’et Imecha" ("Respect your father and your mother"), as the word "Et" in the phrase "Et Abicha" alludes to one’s father’s spouse, and the word "Et" in the phrase "Et Imecha" alludes to one’s mother’s spouse.
After the parent passes on, one is not strictly required to continue respecting the parent’s spouse, but it is certainly proper to do so.
Furthermore, one is obligated to show respect to all his older siblings – both brothers and sisters. This applies even to older siblings with whom one shares only the same father or only the same mother. Even if a younger brother is a Torah scholar, he must show respect to his older siblings. According to some opinions, this obligation continues even after the parents are deceased.
Of course, the respect that is required for one’s older siblings is less than the respect that is required for one’s parent. Thus, for example, Hacham Ovadia Yosef writes in Halichot Olam (p. 163; listen to audio recording for precise citation) that one may call his older siblings by their first name, as opposed to parents, whom one may not call by their name. If an older brother is a Rabbi, the younger siblings should call him "Rabbi," but may use his first name. An example of the type of respect required for an older brother is standing when an older brother is called for an Aliya to the Torah in the synagogue.
The Kessef Mishneh (commentary to the Rambam’s Mishneh Torah by Maran Rav Yosef Karo, author of the Shulhan Aruch), commenting to the Rambam’s discussion in Hilchot Mamrim (end of chapter 6), writes that the requirement to respect older siblings was enacted by the Sages, and does not constitute a Torah obligation. This is as opposed to the requirement to respect stepparents, which is included in the Torah obligation of respecting parents. On this basis, Hacham Ovadia Yosef writes that if a conflict arises between these obligations, the respect owed to a stepparent supersedes the respect owed to an older sibling, as Torah obligations override those enacted by Hazal. And thus if one must choose between respecting one’s stepparent and respecting an older sibling – such if they have conflicting wishes, and one must choose one over the other – he should respect the stepparent.
Summary: The Torah obligation of respecting parents includes respecting a stepparent – one’s parent’s spouse who is not one’s biological parent. There is also a requirement enacted by the Sages to respect one’s older siblings, including both brothers and sisters, and including half-brothers and half-sisters. Nevertheless, one may call an older sibling by their name. Even a younger brother who is a Torah scholar must show respect to his older siblings.