One of the ways in which a wife expresses her respect towards her husband is by honouring and respecting his mother.
The Muslim woman who truly understands the teachings of her religion knows that the person who has the greatest right over a man is his mother, as one notes in many Prophetic Narrations, such as, ‘Aa'ishah (may Allah be pleased with her) asked the Messenger of Allah (peace be upon him), “Who has the greatest rights over a woman?” He said, “Her husband.” She asked, "And who has the greatest rights over a man?” He said, “His mother.” (Reported by al-Bazzar with a hasan [good] isnaad [chain].)
Therefore, a Muslim woman helps her husband honour and respect his mother, by also honouring and respecting her mother-in-law. In this way she will do herself and her husband a favour, as she will be helping him to do good deeds and fear Allah (the Most High), as commanded by Him in the Qur'aan.
At the same time, she will endear herself to her husband, who will appreciate her honour and respect towards his family in general, and towards his mother in particular. Nothing could please a decent, righteous and respectful man more than seeing strong ties of love and respect between his wife and his family, and nothing could be more hateful to a decent man than to see those ties destroyed by the forces of evil, hatred and conspiracy. The Muslim family which is guided by faith in Allah (the Most High) and follows the pure teachings of Islaam is unlikely to fall into the trap of such jaahili (ignorant) behaviour, which usually flourishes in communities today.
A Muslim wife may find herself being tested by her mother-in-law and other in-laws, if they are not of good character. If such is the case, she is obliged and would be meritorious to treat them in the best way possible, which requires a great deal of cleverness, courtesy, diplomacy and repelling evil with that which is better. Thus she will maintain a balance between her relationship with her in-laws and her relationship with her husband, and she will protect herself and her marriage from any adverse effects that may result from the lack of such a balance.
Source: The Ideal Muslimah by Dr. Muhammad Ali Al-Hashimi.