In Laws-Honoring ones Father and mother In-Law:
A man is [Rabbinically] obligated to [slightly] honor his father in-law [and mother in-law]. [Likewise, a wife is obligated to honor her father-in law and mother in-law, and even more so than her husband is obligated to honor her parents, being that honoring his parents in addition to the honor that they deserve on their own right is also an honor for the husband himself. This applies even after the death of one’s spouse.]
To what extent must one honor his in-laws: Some Poskim rule that one is obligated to honor them just as he is obligated to honor other dignified elders, however not to the extent that he is obligated to honor his own father and mother. However, other Poskim rule which one is obligated to honor his father-in-law and mother-in-law to the same extent that one is obligated to honor his father and mother in speech and action.
The matters of honor that one is to show his in-laws:
As stated above, one is not obligated to honor his in-laws in the same forms of respect as one is obligated towards his parents. However, one is to honor them in the following:
1. Standing up for them: One is to stand on behalf of his or her in-laws as one would stand for an elder who would enter the room.
2. Speaking respectfully: One is the speak respectfully with his or her father and mother-in-law and refer to them in an honorary title.
Honor him to sit at the head of table and speak first: One should position his father-in-law to sit at the head of the table and allow him to be the first to speak.
Fearing one’s in-laws:
One is not obligated to fear his in-laws as he is obligated with his parents and the obligation towards one’s in-laws is only regarding matters of honor.
Sitting in their set place:
From the letter of the law, one may sit in the set place designated for his in-laws to sit. Nonetheless, the custom is to be stringent in this matter and not sit in their set place especially in the home of the in-laws, and accordingly one is to be stringent in this matter.
Calling them by name:
From the letter of the law, one may call his in-laws by their first name. Nonetheless, the custom is to be stringent in this matter and not call one’s in-laws by their first name, especially when one is in their presence, and accordingly one is to be stringent in this matter.
A son or daughter in law remains slightly obligated in the respect of their in-laws even after the in-laws have deceased. Hence, we find that the son and daughter-in-law are to follow certain mourning customs after the death of their father or mother-in-law.
Learning Torah on his behalf: When a son-in-law studies Torah in memory of his father-in-law, it elevates the soul of his father.
Kaddish: A son in-law may recite Kaddish on behalf of his father or mother-in-law and even receives precedence over a brother [or father] of the deceased.
Order of precedence:
The honor and respect that one needs to show his father, mother, grandparents, and older brother comes before his in-laws.
Naming after father versus father-in-law:
If one has a son born to him and both his father and father-in-law have passed away, then he is to precede to call his sons name after the name of his father then after the name of his father-in-law.
One’s in-law’s spouse:
There is no obligation for one to honor his in-law’s spouse, such as the spouse of his mother-in-law or spouse of his father-in-law who is not his or her spouse’s biological parent.
 Michaber 240:24; Tur 240; Orchos Chaim 2:5 Alef; Semak Mitzvah 50; Midrash Socher Tov Mizmor 7; Tana Dvei Eliyahu Raba 24; Pesakim Uteshuvos 240:60; Encyclopedia Talmudit Erech Kibud Av Vaeim Vol. 26 p. 476
 Chayeh Adam 67:17; Pischeiy Teshuvah 240:20; Chikreiy Lev Y.D. 3:98; Reshimos Choveres 184 in name of Rebbe Maharash in letter; Encyclopedia Talmudit ibid footnote 1395
Other opinions: Some Poskim rule that it is a biblical obligation to honor one’s in-laws. [See Chareidim Asei 4:10 who implies that it is Biblical; Encyclopedia Talmudit ibid footnote 1394]
 Taz 240:19; Bach 240; Tzeida Ladarech 4:15; Beir Heiytiv 240:17; Pischeiy Teshuvah 240:20; Birkeiy Yosef 240:21-22; Leket Yosher 2:37; Tzeida Laderech Mamar 1 4:15; Implication of Michaber Y.D. 374:6 that the husband is to join in mourning for his wife’s father and mother due to their honor. [Pischeiy Teshuvah ibid]; See Hagahos Rav Akiva Eiger 240
 The reason and source: This is learned from the fact that Scripture [Shmuel 1 24:11] relates that David referred to Shaul who was his father-in-law as his father [i.e. Avi Reah Gam Reah]. [Taz 240:19; Tur 240; Beis Yosef 240 in name of Orchos Chaim 5; Semak Mitzvah 50; Midrash Socher Tov Mizmor 7; Beir Hagoleh 240:47; Orchos Chaim ibid; See Hagahos Chasam Sofer 240] Alternatively, it is learned from the fact that Moshe kissed his father-in-law Jethro when he saw him, and hence from here we learn that one should honor his father in-law. [Mechilta Parshas Yisro 1; Biur Hagr”a 240:32] Alternatively, this is learned from the fact that a man and wife are considered like one body, and hence the father and mother of one’s spouse are considered like one’s own father and mother. [Chareidim Asei 4:10] Alternatively, since the verse approximates the prohibition of adultery to the command of honoring one’s parents, we can learn that one who marries a woman and does not respect her father and mother it as if he is being promiscuous with her his whole life. [Tana Dvei Eliyahu Raba 24]
 Birkeiy Yosef 240:21-22; Leket Yosher 2:37; Chareidim Asei 4:10; Meishiv Devarim 140
 Machaneh Chaim 2 C.M. 27; Meishiv Halacha 2:146
 See Encyclopedia Talmudit ibid
 Shach 240:22; Bach 240; Beir Heiytiv 240:17, Birkeiy Yosef 240:21; Pischeiy Teshuvah 240:17; Chayeh Adam 67:24; Chikreiy Lev Y.D. 3:98; Ikarei Hadat 26; Bris Olam of Chida on Sefer Chassidim 345; Binyan Olam Y.D. 47; Tzafichis BeDevash 54; Bigdei Yom Tov Y.D. 33; Minchas Elazar 3:33; Betzel Hachochmah 1:69; 2:90; Yechaveh Daas 6:51
 See Chayeh Adam who writes elders and dignified people; See Pesakim Uteshuvos 240:60 footnote 507
 The reason: This is proven from the fact that the Michaber rules that a wife is not obligated to honor her father being that she must honor her husband, and if it were true that the husband himself is obligated to honor her father than she too would be obligated to honor him, as is the law with regards to ones honoring his father over his mother being that she is too commanded to honor her husband. [Pischeiy Teshuvah ibid]
 Midrash Tehillim Mizmor 7, brought in Bach 240; Levush 240; Chareidim Asei 4:16
 Binyan Olam Y.D. 47; Minchas Elazar 3:33
 As can be seen from the fact that David refer to his father-in-law soul as father. [See also Ramban Bereishis 31:46
 Ikarei Hadat 26; Bris Olam of Chida on Sefer Chassidim 345; Betzel Hachochmah 1:69; 2:90; Yechaveh Daas 6:51
 Sefer Yosher Hori 17:16
 Sefer Yoshere Hori 17:16
 Michaber Y.D. 374:6
 See Yesod Veshoresh Havoda Shaar Hakolel 15
 Mishmeres Shalom Kuf 55; Toras Menachem Tziyon 2:381, printed in Shulchan Menachem 5:296; See however Zera Emes 2:148 that it is better that the brothers of the deceased recite Kaddish for him; See Pnei Baruch 34:24; See Pnei Baruch 34:25; Piskeiy Teshuvos 132:30
Other opinions: Some Poskim write that the saying of Kaddish is exclusively for the descendants of the deceased and not a son in law. [Sdei Chemed Mareches Aveilus 159]
 Minchas Elazar 3:33
 Ikarei Hadat 26; Sdei Haaretz 3:22 for this reason Aron was punished by having his two children die as he preceded to call his first son after his father in law Nadav rather than his father; See Divrei Yechezkal 33; Yosher Hori 17:7
 Leket Yosher 2:37; Rit Vyaal 59; Chasam Sofer E.H. 1:38; Encyclopedia Talmudit ibid footnote 1400-1401