Gemara

Question (Rava): [If Ploni was not Me'arev and died on Shabbos,] can his heir be Mevatel?

Perhaps only one who could have been Me'arev can be Mevatel. The heir was unable (he did not own the property before Shabbos);

Or, perhaps he can be Mevatel, for an heir is like the leg (continuation) of the one he inherited from!

Kesuvos 103a (Beraisa): When Rebbi passed away, he instructed his sons "be careful to honor your mother..."

Question: Why did he need to tell them this? This is mid'Oraisa!

Answer: He referred to his wife. (She was not their mother.)

Question: Also this is mid'Oraisa!

(Beraisa): "Kaved Es Avicha v'Es Imecha" - 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.

Kidushin 30a (Rav Yehudah): One must teach Torah (to two generations,) like the case of Zevulun ben Dan, whose grandfather taught to him.

Question (Beraisa): "You will teach your sons" - not your grandsons.

Answer: Rav Yehudah holds like another Tana who [uses "sons" to exclude daughters and] learns from "you will make known to your sons and grandsons."

Bava Basra 91b (R. Elazar): "Va'Yasem Kisei l'Em ha'Melech" was for Rus, the matriarch of the kingship.

128a (Mar bar Rav Ashi): One can testify about his grandfather.

The Halachah does not follow Mar bar Rav Ashi.

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 gave all his property to his wife, and all his sons were from other wives, what is the law?

Sotah 49a: Rav Acha bar Yakov raised his daughter's son, Yakov. When Yakov grew up, Rav Acha bar Yakov asked him to get him water to drink.

Yakov: I am not your son! (I need not honor you.)

Rishonim

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

Rambam (Hilchos Mamrim 6:15): One must honor his father's wife, even if she is not his mother, as long as his father is alive. This is included in Kibud Av. Similarly, one must honor his mother's husband only as long as his mother is alive.

Question (Shitah Mekubetzes 103a DH Hani): The Gemara connotes that mid'Rabanan, one must honor his step-mother after his father dies. If so, why did Rebbi need to command his children about this?

Answer #1 (Shitah): An early version of Rashi explains that there is no Mitzvah after he dies.

Answer #2 (Hafla'ah 103a DH Sham Gemara): The Rema (YD 240:7) says that if the son is his father's Rebbi, he may pardon his honor and serve his father. This implies that he is exempt. All the more so, a Nasi is exempt! Rebbi needed to command his children because one of them would be Nasi.

Rashbam (131b): If a man gave all his property to his oldest son or wife, he only made him or her an overseer, so that his [other] children will honor him or her. We ask about a step-mother, for children are not commanded so much to honor a step-mother [after the father's death]. It is learned from a Ribuy.

Rashi (Bereishis 46.1, citing Bereishis Rabah Vayigash 94:5): Yakov offered Korbanos "lEi'lokei Aviv Yitzchak", and not to lEi'lokei Avi Aviv, for one must honor his father more than his grandfather.

Maharik (30/44 in Venice edition): There is no source that one must honor his grandfather, just there is a custom to say Kaddish for a grandfather. Grandchildren are like children only regarding Peru u'Rvu, like it says in Bechoros. (We find this in Yevamos 62b - PF.) Mar bar Rav Ashi permits a grandson to testify for his grandfather! Even though the Halachah does not follow him, he must be able to resolve the Beraisa that says that grandchildren are like children.

Rebuttal #1 (Teshuvas Rema 118): Bereishis Rabah obligates honoring one's father more than his grandfather. This shows that one must honor his grandfather. All the more so, if the grandfather raised the grandchild, 'one who raises an orphan in his house, it is as if he fathered him' (Sanhedrin 19b), or if he hired a Rebbi for him, he must honor him.

Rebuttal #2 (Bach DH Kosav): It is logical that one must honor his grandfather more than his father-in-law. Regarding testimony it says "Avos Al Banim." It forbids only fathers, but not grandfathers, who are another generation away. We learn Kibud Av v'Em from "Kaved Es Avicha". "Es" includes even step-parents and one's older brother, all the more so a grandfather, who is called a father. Hash-m told Yakov "Ani Hash-m Elokei Avraham Avicha". Also, a grandfather must teach Torah to his grandchild (Kidushin 30a). Surely he is like a father also regarding honor! Also, the reason for Kibud Av is because he is a partner in the formation of the son. This applies also to the grandfather! Tosfos (Yevamos 3a DH mi'Kamei) says that regarding Ervah, a daughter-in-law is considered a closer relative than one's brother's wife, for a son is like the leg of his father. One's granddaughter is considered his own relative more than his sister. Likewise, regarding honor we can say that a grandson is like the leg of his father. Honor is unlike testimony.

Poskim

Shulchan Aruch (YD 240:21): One must honor his father's wife, even if she is not his mother, as long as his father is alive. One must honor his mother's husband as long as his mother is alive. He need not honor her [or him] after the death of his father [or mother]. Nevertheless, it is proper to honor them after death.

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

Birkei Yosef (16): One must honor his father's wife even if his mother objects.

Shulchan Aruch (24): One must honor his father-in-law.

Source (Beis Yosef DH u'Mah she'Chosav Chayav): David called Sha'ul "my father" (Shmuel 1:24:12) because Sha'ul was his father-in-law.

Bach (DH v'Chayav): The same applies to his mother-in-law. R. Yonah connotes that one need honor a father-in-law only like one honors Chachamim, e.g. standing up for them. It seems that the Tur agrees. However, Medrash Shochar Tov (Mizmor 7) equates it to Kibud Av. This connotes one must do everything for his father-in-law that he does for his father. Why did the other Mechabrim omit this entirely? Perhaps it is because R. Yehudah says that David called Sha'ul "my father", but Chachamim say that he said this to Avner, who was his Rebbi. Nevertheless, he said to Sha'ul "also see", therefore one must show some respect.

Birkei Yosef (20): If the Poskim omitted it, this shows that one need not honor a father-in-law at all! Perhaps the Tur holds that it is unreasonable to make such a large argument, so Chachamim agree that some honor is required. The Mechilta expounds "va'Yishtachu va'Yishak Lo" (Shmos 18:7) to teach that Moshe bowed to and kissed Yisro, for one must show some honor to his father-in-law. It seems that all agree to this.

Birkei Yosef (22): A woman's Mitzvah to honor her father-in-law is greater than a man's Mitzvah, for she must honor her husband. However, she is in her husband's Reshus, so her ability to fulfill [even Kibud Av v'Em] is limited.

Rema: Some say that one need not honor his grandfather. I disagree. However, one must honor his father more than his grandfather.

Tana d'vei Eliyahu (24 DH Kaved, cited partially in Shirei Berachah 16): Kibud Av v'Em is written near the Isurim of adultery and kidnapping to teach that if a woman does not honor her father-in-law and mother-in-law in their old age, it is as if the entire marriage was adultery. If a man's children do not honor his father and mother in their old age, it is as if he kidnapped.

Birkei Yosef (17): Shevus Yakov (2:94) includes a Mitzvah to honor a grandmother from "va'Yasem Kisei l'Em ha'Melech."