In describing the profound sense of grief felt by Yaakov Avinu after he became convinced that his precious son Yosef had been attacked and killed by a wild animal, the Torah states that he refused to be comforted, despite the efforts of all his sons and all his daughters who tried to console him (בראשית ל"ז:ל"ה). In view of the fact that the Torah specifically records the birth of only one daughter to Yaakov (שם ל':כ"א), many Meforshim are troubled by the reference in the Posuk in our Parsha (שם ל"ז:ל"ה) to "all his daughters" (וכל בנתיו), which obviously implies the presence of more than one daughter. Ibn Ezra (שם בד"ה וכל) understands this as a reference to his (one) daughter along with (at least) one granddaughter that he may have had then; Rabbeinu Bechaya (שם בד"ה וכל) similarly learns that this refers to his daughter Deenah, along with one of Yaakov's granddaughters, Serach, the daughter of Asher, who is actually singled out later in the Torah (שם מ"ו:י"ז) when other females are not mentioned, and the Ramban on the Posuk in our Parsha (שם ל"ז:ל"ה) mentions this idea as well. The Torah actually makes a similar reference to Yaakov's daughters (in the plural) later on (שם מ"ו:ז'); the Daas Zekeinim MiBaalei HaTosafos there (שם בד"ה בנותיו) offers different possible interpretations of this, while Ibn Ezra there (שם בד"ה בנותיו), interestingly, suggests that this reference is to the maids who were raised with Yaakov's (only) daughter Deenah and were thus themselves considered like daughters of Yaakov, even though they were really maids; it is clear from the context of that Posuk (שם), though, that the reference there cannot be to granddaughters, while in the Posuk in our Parsha (שם ל"ז:ל"ה), the reference can be to granddaughters.
Rashi in our Parsha (שם בד"ה וכל), however, cites two different opinions about this from the Midrash in Bereishis Rabbah (פרשה פ"ד סימן י"ט). One is that a twin sister was born with each of Yaakov's twelve sons; Yaakov thus had plenty of daughters who tried to console him, although no direct reference is made to any of them in the Torah. This view is also found in the Pirkei DeRabbi Eliezer (פרק ל"ו,ל"ט), as well as in Tosafos in Bava Basra (דף קכ"ג. בד"ה תיומה). The other view is that the reference is to Yaakov's sons' wives, that is, his daughters-in-law, who came from other nations, as implied as well by the Midrash in Bemidbar Rabbah (פרשה ב' סימן ז'). The Midrash in Bereishis Rabbah (שם), as cited by Rashi (שם), states that these daughters-in-law are called Yaakov's daughters because a person does not refrain from calling his daughters-in-law "daughters". This view is also found in the Targum Yonasan Ben Uziel on the Posuk in our Parsha (שם), as well as in the commentary of the Ramban (שם), who quotes other Pesukim in which daughters-in-law are called daughters, and then adds that calling someone a daughter is an expression of love.
The Revid HaZahav, in his commentary on the Posuk in our Parsha (שם), notes that since one's daughter-in-law is considered like one's daughter, it follows that the reverse relationship is present as well, meaning that one's father-in-law is like one's father, and consequently, one must honor one's father-in-law as one must honor one's own father. In discussing the Posuk later in the Torah (שמות י"ח:ז') where Moshe Rabbeinu meets and warmly greets his father-in-law Yisro, the Mechilta (שם פ' יתרו פרשה א') explains that Moshe actually bowed to Yisro and kissed him, and thus derives from there that one must be prepared to show great honor to his father-in-law, as Moshe did. Similarly, the Yalkut Shimoni (חלק ב' רמז קל"ג), commenting on a Posuk in Shemuel Aleph (כ"ד:י"ב) which records a discussion, according to one view there, between Dovid and Shaul, notes that since the Posuk (שם) states that Dovid referred to Shaul, who was his father-in-law (as Dovid was married to Shaul's daughter Michal), as אבי"," my father, we may derive that a person is obligated to honor his father-in-law just as he is obligated to honor his own father. The Tur (יורה דעה סוף סימן ר"מ) thus rules, referring to the Posuk in Shemuel Aleph (שם), that one is obligated to honor one's father-in-law, and the Shulchan Aruch (שם סעיף כ"ד) also rules accordingly. The Bach, commenting on the Tur (שם בד"ה כתב הרמב"ם ממזר...), writes that the same is true of honoring one's mother-in-law. The Taz (שם ס"ק י"ט) and the Be'er Heitev (שם ס"ק י"ז) also write that one must honor one's mother-in-law as well, as does the Pischei Teshuvah (שם ס"ק כ'), who notes that the Shulchan Aruch himself seems elsewhere (שם סימן שע"ד סעיף ו') to equate a mother-in-law with a father-in-law in terms of this obligation to display honor.
The Bach, in his aforementioned commentary on the Tur (םש), however, writes that it would appear that the obligation to honor one's in-laws implies merely that they must be honored the way any older people should be honored, namely, by standing up out of respect when they enter the room and by displaying other such forms of honor. He
admits, though, that this position is somewhat difficult in light of the Midrash, cited above in the name of the Yalkut Shimoni (םש), which equates honoring one's in-laws with honoring one's parents, and thus implies that one must honor one's in-laws in the exact same fashion in which one must honor one's parents, but he finds accepting that position very problematic because the Tur (םש) doesn't really say that the obligation to honor one's in-laws is that strict and all-encompassing, and other Poskim, as he asserts (שם), do not mention this Halacha at all. He thus explains (םש) that there really is no proof at all from that Midrash, because according to one opinion there, which happens to be the majority opinion, Dovid was not even addressing Shaul when he used the word "אבי," my father, and thus one cannot derive from there that one must honor one's in-laws the same way one honors one's own parents; for this reason, other important Poskim is do not quote this Halacha to begin with. He concludes, though, that there is nevertheless an obligation to honor one's in-laws in the way that one honors other important older people. This position is accepted by the Shach (שם ס"ק כ"ב), by the Pischei Teshuvah (םש), and by the Chayei Adam (כלל ס"ז סעיף כ"ד), among others.
The Chida, in his Sefer Birkei Yosef on the Shulchan Aruch (שם ס"ק כ'), wonders, after citing the Bach (םש), why, if other Poskim omitted this entire Halacha, the Tur (םש) ruled that there is a special obligation to honor one's in-laws, especially since the opinion that requires it in the Midrash in the Yalkut Shimoni (םש) is the minority position. He suggests two answers (םש); the first is that since there is one opinion that requires one to honor one's in-laws the same way one honors one's parents, the other opinion most likely is not that there is no obligation for one to honor one's in-laws at all, but rather that there is such an obligation, but it is just not as encompassing as the obligation to honor one's parents. He then answers that the aforementioned Mechilta (םש) also represents a valid source for this obligation, since it specifically says that one should honor one's father-in-law, although the requirement is not the same as that to honor one's father; he asserts that this view is in agreement with the majority view in the Midrash cited in the Yalkut Shimoni (םש), and thus everybody holds that there is some obligation to honor one's in-laws. The only question he leaves unanswered is why other Poskim omitted this entire Halacha. Similarly, the Torah Temimah, in his commentary on Parshas Yisro (שמות שם אות ח'), asks why none of the Poskim cite the aforementioned Mechilta (םש) about how Moshe treated Yisro as a source for this Halacha that one must honor one's in-laws; he asserts that even if no proofs can be brought from the Midrash in the Yalkut Shimoni (םש), the Mechilta (םש) should be accepted as a perfectly valid source, and he leaves the matter as a question. He also states (םש) that the decision of the Bach (םש) that the obligation to honor one's in-laws means only that they must be honored like other older people must be honored is incorrect, since we see from Moshe's behavior towards Yisro that one must display much greater honor towards one's father-in-law than one would towards an ordinary older person.
The Sdei Chemed (כללים, מערכת הכ"ף כלל ק"כ), however, who cites the above mentioned view of the Birkei Yosef (םש) and questions it, discusses the role of the Mechilta (םש) in the development of this requirement to honor one's in-laws, and he concludes that it apparently is not really a significant source in this particular discussion, because nobody, not even the Tur (םש), quotes it as a major source. Rav Ovadyah Yosef (שו"ת יחוה דעת חלק ו' סימן נ"א) points out that the Mechilta (םש) does not really formulate any specific obligation, but rather states that one should be prepared to honor one's in-laws, as if to imply that such behavior is proper in terms of Derech Eretz and manners, but it is not required by Halacha, and he adds that bowing to and kissing one's in-laws, as Moshe did, is not really obligatory. He concludes (םש), after bringing an elaborate proof, that the Bach (םש) is correct in ruling that the obligation to honor one's in-laws is similar to the obligation to honor any older person, even if he is not a Talmid Chochom, such as by standing up in their presence (עיין בשולחן ערוך יו"ד סימן רמ"ד סעיף א' וברמ"א שם, ועיין עוד בערוך השולחן שם סעיף י').
It should be noted that the Birkei Yosef (שם סימן ר"מ ס"ק כ"ב) writes that a woman is certainly required as well to honor her in-laws to the greatest extent possible, and that in a certain sense, she has an even greater obligation than a man, because she is obligated to honor her husband, as the Rambam (פרק ט"ו מהל' אישות הלכה כ') notes, and she honors her husband by honoring his parents; Rav Ovadyah Yosef (םש) rules this way as well. It should also be noted that the Mishnah in Kesubos (דף ע"ב.) states that a woman who curses her in-laws may be divorced without being given her Kesubah payment; the Rambam (פרק כ"ד שם הלכה י"ב, ט"ז) and the Shulchan Aruch (אבן העזר סימן קט"ו סעיף ד', ועיין שם ברמ"א ובשאר פוסקים) rule accordingly. Interestingly, though, the Chida, in his Sefer Bris Olam, commenting on the Sefer Chassidim (סימן שמ"ה), writes that whereas there is an obligation to display honor (כבוד) to one's in-laws, there is no obligation to fear, or to demonstrate awe (מורא), towards them.
The Sefer Chareidim (פרק י"ב, אות ג'-י') understands that the obligation to honor one's in-laws is MideOraisa, noting that since a husband and a wife are considered as one person, the husband's parents are also the wife's parents, and the wife's parents are also the husband's parents; consequently, each must honor the other's parents as his or her own are honored. He then quotes a version of a passage in the Tanna DeBei Eliyahu (אליהו רבא פרק כ"ו) which indicates that one who marries a woman who does not honor his parents when they are older is considered like a life-long adulterer; he adds later (פרק ט"ז, אות ג'-י"ב) that this obligation to show honor pertains to both one's manner of speech as well as to providing actual service. The Vilna Gaon, in a letter he wrote to his family members when on his way to Eretz Yisrael (אגרת הגר"א), urges them to honor their in-laws,
especially when they have reached an old age. The Kitzur Shulchan Aruch (סימן קמ"ג סעיף כ') also rules that one must honor one's in-laws; the Sefer She'arim Metzuyanim BeHalacha (שם ס"ק י"ד) notes that even if one's parents and one's in-laws do not get along, and one's parents instruct him not to visit his in-laws, he need not obey them, because he is obligated to honor his in-laws as well. He also writes (םש) that one must take into account one's in-laws' feelings in certain cases when naming one's child. It is thus clear that one must treat one's in-laws with great respect, even if the obligation to honor them is not quite the same as the obligation to honor one's own parents.