Gemara

Bava Basra 131b (Rav Yehudah): If a man wrote a document giving all his property to his wife, or to his oldest son, he only made him an overseer.

Question: If he wrote all his property to a younger son, what is the law?

(Rav Chanilai bar Idi): Even if he wrote to a baby in a crib, the son is only an overseer.

Question: If he wrote to a daughter and he has sons, what is the law?

Kesuvos 103a - Question: Why did Rebbi need to instruct his sons to honor his wife after he dies? This is mid'Oraisa!

(Beraisa): "Kaved Es Avicha v'Es Imecha Lema'an Ya'arichu Yomecha" - the first "Es" includes your father's wife. The second includes your mother's husband. The 'Vov' (in v'Es) includes your older brother.

Answer: The Torah commands only in the parent's lifetime.

Rishonim

Rif and Rosh (Kidushin 12b and 1:45): "Kaved Es Avicha v'Es Imecha" includes your father's wife and your mother's husband. The 'Vov' (in v'Es) includes your older brother. This is only in the parent's lifetime, but not after death.

Rambam (Hilchos Mamrim 6:15): One must honor his father's wife (and his mother's husband), even if he (she) is not his parent, as long as his father (mother) is alive, but not after he (she) dies. This is included in Kivud Av v'Em. Mi'Divrei Sofrim, one must honor his older brother like Kivud Av.

Kesef Mishneh: The Rambam considers honoring step-parents to be explicit in the Torah because it is expounded from "Es". He calls honoring an older brother 'mi'Divrei Sofrim'; it is included from an extra Vov.

Question (Lev Some'ach, Sefer ha'Mitzvos Reish Shoresh 2 DH va'Anochi): What is the difference? "Es" and "Vov" are both Ribuyim (inclusions)!

Answer #1 (Lev Some'ach): The Rambam would have said that also honoring step-parents is mi'Divrei Sofrim had the Gemara not said that it is mid'Oraisa.

Answer #2 (Shitah Mekubetzes Kesuvos 103a DH Vov): We expound "Es" everywhere. Here it includes step-parents; it is part of Kivud Av v'Em, and applies only in their lives. The Vov includes an older brother; this is an independent Mitzvah.

Note: The Rambam (Sefer ha'Mitzvos Shoresh 2) and Sefer Chasidim (1015) say that honoring an older brother is learned from "Es". R. Yerucham (Nesiv 2:5) says that our Gemara learns from "Es" (it seems that this was his text).

Ramban (beginning of Shoresh 2 DH ha'Ikar): BaHaG did not count honoring step-parents and an older brother to be separate Mitzvos because they are due to Kivud Av v'Em. There is no Mitzvah after the death of the parents.

Megilas Esther (Shoresh 2, DH uv'Inyan): The Rambam did not say that honoring an older brother applies only in the father's lifetime. It seems that he holds that it applies even after his death.

Rashbam (Bava Basra 131b): If a man gave all his property to his wife (or his oldest son), he only made her (him) an overseer, so that his (other) children will honor her (him). We expound that one must honor his older brother. We know that a man does not want his daughter to be disgraced and need to beg. We ask whether he wants her brothers to honor her.

Poskim

Shulchan Aruch (22): One is obligated to honor his older brother, whether he is a paternal or maternal brother.

Source: Teshuvas ha'Rosh (15:6, cited in Beis Yosef DH u'Mah she'Chosav Chayav): The Torah wrote the Ribuy to include an older brother regarding the mother, to teach that it applies to a maternal brother, and all the more so to a paternal brother.

Darchei Moshe (7 DH uvi'Teshuvas): The Maharik (44) and Ramban say that the obligation to honor step-parents and an older brother are due to Kivud Av. It is not clear whether or not the father can pardon his honor (to exempt his son from honoring them).

Pischei Teshuvah (18): Keneses Yechezkeil says that the Ramban and Rambam argue about whether or not the Mitzvah to honor an older brother is due to Kivud Av, and does not apply after the father dies.

Birkei Yosef (17): The Rambam, Rashi, SMaG, Ran (Kidushin 12b), Tur, Shitah Mekubetzes and others all obligate honoring an older brother even after the father dies. We do not abandon them due to the Ramban's Safek.

Shirei Berachah (12): Mishkenos Yakov says that the Mitzvah to honor a step-father is surely no greater than the Mitzvah to honor a step- mother. We learn about honoring an older brother from (the Vov in) the same word ("v'Es") that teaches about a step-father, so also this should not apply after death! However, the kinship of a brother does not go away after the father dies. We find that Rami bar Yechezkeil said 'Do not heed these rules that my brother quotes in the name of Shmuel' (Kesuvos 21a), even though his brother (Rav Yehudah) was older (Sanhedrin 80b). He would not speak this way if the Mitzvah was mid'Oraisa! I say that this is not a proof. Surely, one need not honor a brother as much as a father. Further, he was not in front of his brother when he said this.

Gilyon Maharsha (11, citing Hilchos Ketanos 1:28 (it seems that the reference is wrong)): The Mitzvah is to honor only the oldest brother.

Beis Lechem Yehudah (12): The Ari Zal says that the Mitzvah is only to honor the oldest brother, and certainly not an older sister.

Shevus Yakov (1:76, cited by Pischei Teshuvah 19): There is no source in Shas or Poskim for a Mitzvah to honor an older sister. Even though Rachel was punished for speaking before Leah, we do not learn from Agadata. Further, this is not a matter of honor. Rather, it is a disgrace to speak in front of someone greater than himself, even if he is not a relative (Avos 5:7). The Mitzvah to honor an older brother applies only to the oldest, who inherits his parents' greatness. Nevertheless, we find that Yisachar and Zevulun would not speak in front of their older brothers (Rashi Bereishis 49:5).

Rebuttal (of the last three entries - Birkei Yosef 17): The Rambam (Hilchos Tzara'as 16:10) connotes that it is a Mitzvah to honor an older sister. (He says that we should remember how Miryam was punished... even though she was Moshe's older sister...) The Bach brings a Mitzvah to honor a mother-in-law, even though the Gemara mentions only a father-in-law. We must say that a mother-in-law is included. Likewise, perhaps the Mitzvah to honor an older brother includes the oldest sister! R. Yonah includes a Mitzvah to honor uncles due to Kivud Av v'Em. Shevus Yakov himself (2:94) includes a Mitzvah to honor a grandmother from "Va'Yasem Kisei l'Em ha'Melech", i.e. for Rus, the matriarch of the kingship (Bava Basra 91b). The Ramban holds that there is a Mitzvah to honor all older brothers. The correct text of the Ari Zal explicitly says that the Mitzvah is to honor all older brothers, even after the father died. This includes an older sister. Beis Yehudah (YD 28) says that since women must stand for Zekenim, others must stand for a Zekenah. Likewise, since a woman must honor an older brother, brothers must honor an older sister.

Shirei Berachah (14): Maharhal learns from the Rashbam that there is no Mitzvah to honor an older sister. He explains that an oldest son was merely made an overseer, so that the other children will honor him. He did not suggest this at all regarding a daughter! This is not a proof. Once we said that even an infant brother was made an Apotropos, we retract from saying that it is due to the Mitzvah to honor an older brother.

Rema: This is even if the younger brother is a Chacham and greater in Torah. This seems correct, unlike those who disagree.

Darchei Moshe (7 DH Kosav): Binyamin Ze'ev says that a Chacham comes before his older brother. His proofs are invalid. Rather, he must honor his older brother, just like all agree that a Chacham must honor his father unless the father is his Talmid.