Is there a Halachic requirement to show respect to one’s parents-in-law, and, if so, how is this manifest?
The Shulhan Aruch indeed codifies an obligation to respect one’s parents-in-law, though this obligation differs from the Misva of honoring one’s parents. For one thing, while respecting one’s parents clearly constitutes a Torah command, there is some question regarding the status of the obligation to respect in-laws. Some authorities maintain that respecting in-laws is also a Biblical obligation, as evidenced by the respect that Moshe showed his father-in-law, Yitro (see Shemot 18:7). This appears to have been the view of the Vilna Gaon (Rabbi Eliyahu of Vilna, 1720-1797), who wrote in his famous letter to his family that they should show respect to their in-laws "as is written in the Torah." Others, however, claim that the requirement to respect one’s parents-in-law was enacted by Hazal, and does not constitute a Biblical obligation.
Practically speaking, Halacha requires one to stand in honor of a parent-in-law when he or she comes within four Amot (approx. 6 feet) of him. For a parent, one must stand as soon as the parent comes within view, even if he or she is still at a distance; this applies also to one’s Rabbi. In-laws, however, are treated in this respect like Torah scholars (other than one’s personal Rabbi), for whom one stands only when they are within four Amot of him.
One should make a point of visiting his parents-in-law regularly, just as one should visit his parents.
It is forbidden to call one’s in-laws by their first names. Some authorities write that one should call them "Mother" and "Father" just as he refers to his own parents. Indeed, King David referred to Shaul, his father-in-law, as "Abi" ("My father" – see Shemuel I 24:11). Others disagree, but according to all views, one should not call parents-in-law by their first names.
When one’s in-laws eat at his home, he should have his father-in-law sit in the most distinguished place at the table as a demonstration of honor. One should also instruct his wife to first serve her father, so that she fulfills the Misva of honoring her father, and he fulfills the Misva of honoring his father-in-law.
If one’s father-in-law or mother-in-law passes away, Heaven forbid, and leaves no one behind to recite Kaddish, he should say Kaddish to elevate the soul. If his parents are still alive, he should receive their permission to recite Kaddish for his in-law.
In principle, one is not required to respect his in-laws if they are Resha’im (wicked people), though one should not take the liberty to decide for himself whether they indeed belong in that category. One should rather consult with a competent Halachic authority before reaching such a conclusion.
Summary: Halacha obligates one to respect his parents-in-law. This includes standing in their presence, visiting them regularly, not calling them by their first name, and having one’s father-in-law sit at the most prominent place at the table.
