What is the duty of a man and a woman regarding their in-laws. It is found that many of them, although they have very good Islamic etiquette, etc., still hate their in-laws for no reason. Please explain in light of Quran and Hadis about the duty of daughter/son to their in-laws in Islam.

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Among the most important obligations upon a Muslim is to be kind to his parents.  Allah Says (interpretation of meaning): {Worship Allâh and join none with Him in worship, and do good to parents,….}[4:36].

The Prophet (Sallallahu Alaihi wa Sallam) said thrice, "Shall I not inform you of the biggest of the great sins?" We said, "Yes, O Allah's Apostle" He said, "To join partners in worship with Allah; to be undutiful to one's parents." The Prophet sat up after he had been reclining and added, "And I warn you against giving forged statement and a false witness; I warn you against giving a forged statement and a false witness." The Prophet kept on saying that warning till we thought that he would not stop".

Not taking care of parents is being undutiful to them, especially when they become aged and need much support from their children.  Allah Says (interpretation of meaning): {…If one of them or both of them attain old age in your life, say not to them a word of disrespect, nor shout at them but address them in terms of honour.}[17:23].

The above is the duty of every person towards his parents.

As for the right of a husband, a wife is obliged to obey her husband.  The Prophet (Sallallahu Alaihi wa Sallam) said: “If a woman performs the five daily prayers, fasts the month of Ramadan, keeps her chastity and obeys her husband, it will be said to her: enter Paradise through any of its gates you wish.” [Ahmad]

The right of a wife on her husband is to deal and behave with her kindly and pleasantly.  Allah Says (interpretation of meaning): {…And they (women) have rights (over their husbands as regards living expenses, etc.) similar (to those of their husbands) over them (as regards obedience and respect, etc.) to what is reasonable,….}[2:228].  He also Says: {….And live with them honourably…..}[4:19]. 

As for the rights of one's in-laws they have only the rights of cognation and neighbourhood.  There is no extra right for them.  In spite of this fact, it is preferable for a husband and wife to keep good ties and behave kindly with their in-laws to maintain a good society.  Being good with in-laws is the main source to develop love and affection in family life.

Allah knows best.