The Yalkut Shimoni states: “David told Shaul, ‘My father, you shall surely see the corner of your coat in my hand’” (which means that David called Shaul his father). Our Sages derived from here that one is obligated to honor one’s father-in-law just as one is obligated honor one’s father, for the verse equates one’s father-in-law to one’s father. The reason for this is because one’s wife is considered an actual part of one’s self and just as one’s wife is obligated to honor her father, the husband is obligated to honor her father as well. The Tur and Shulchan Aruch (Yoreh De’ah, Chapter 240) rule accordingly that one is indeed obligated to honor one’s father-in-law.
Likewise, one must certainly honor one’s mother-in-law. The Mishnah in Masechet Sotah (49b) states that one of the signs of the period immediately prior to the arrival of the Mashiach is that daughters-in-law will rise up against their mother-in-law and others have a different version in the Mishnah that sons-in-law will rise up against their fathers-in-law. Based on this, nowadays when it is common for brazen words to be exchanged between daughter-in-law and mother-in-law or between son-in-law and father-in-law, this is indeed a sign that the end of our exile is near. Although the parents of the husband or wife may sometimes not treat their son-in-law or daughter-in-law properly, it is nevertheless forbidden to act audaciously towards them and one must always treat them respectfully. If the relationship with one’s in-laws causes strife or aggravation, the couple should consult an outstanding Torah scholar who has experience with these issues regarding how to proceed.
Indeed, Hagaon Rabbeinu Yaakov Emdin writes a powerful lesson (in his Responsa She’elat Ya’abetz, Chapter 32) that the common hatred between mother-in-law and daughter-in-law usually exists when they both live in the same house, for this naturally causes strife. However, if they are not so near one another, it is possible that they will live with peace and harmony.
The following are the words of the Tur: “One is obligated to honor one’s older brother just as one is obligated to honor one’s father.” The Tur then continues, “One is also obligated to honor one’s father-in-law, as the verse states, ‘My father, you shall surely see.’” The Bayit Chadash infers from the fact that regarding the honor of one’s older brother, the Tur writes that one should honor him like one honors one’s father whereas regarding the honor one affords one’s father-in-law, the Tur only writes that one must honor him, that the obligation to honor one’s father-in-law is not as great and one need not honor him exactly as one honors one’s father; rather, one only needs to rise before him and honor him the way one would honor an elderly person.
Thus, halachically speaking, although one is obligated to honor one’s father-in-law by not calling him by his first name, rising before him, and the like, nevertheless, one is not obligated to honor him exactly as one would honor his parents by providing them with all of their needs (as we have discussed by the laws of honoring one’s parents) and honoring him minimally, as one would elderly people, is sufficient.
Summary: One is obligated to honor one’s father-in-law by rising before him when he enters one’s four Amot (approximately a six-foot radius) just as one would honor elders. However, one is not obligated to honor him exactly as one is obligated to honor his father. Maran zt”l writes that some customarily kiss the hands of their father-in-law as a show of respect and honor and this is indeed a worthy custom. This is especially true if one’s father-in-law is a Torah scholar and performs praiseworthy deeds. Similarly, one must also honor one’s mother-in-law appropriately. Likewise, a woman should honor her father-in-law and mother-in-law to the best of her ability.
One may not call one’s father-in-law or mother-in-law by their first names; rather, one should call them “Abba” and “Ima” respectively as some customarily do or one should add on a respectful title to their names such as “Rabbi” and the like. Alternatively, one can avoid addressing them directly altogether.