According to Madhhab:
Are son-in-laws regarded as Maharim to their mother-in-laws? Are father-in-laws Maharim to their daughter-in-laws? What kind of a relationship should one share with in-laws?
In the Quraan Sharif Allah Ta'ala has clearly mentioned or defined those women with whom Nikah is not permissible. The verse of Surah Nisaa is quoted below,
حُرِّمَتْ عَلَيْكُمْ أُمَّهَاتُكُمْ وَبَنَاتُكُمْ وَأَخَوَاتُكُمْ وَعَمَّاتُكُمْ وَخَالاَتُكُمْ وَبَنَاتُ الأَخِ وَبَنَاتُ الأُخْتِ وَأُمَّهَاتُكُمُ اللاَّتِي أَرْضَعْنَكُمْ وَأَخَوَاتُكُم مِّنَ الرَّضَاعَةِ وَأُمَّهَاتُ نِسَآئِكُمْ وَرَبَائِبُكُمُ اللاَّتِي فِي حُجُورِكُم مِّن نِّسَآئِكُمُ اللاَّتِي دَخَلْتُم بِهِنَّ فَإِن لَّمْ تَكُونُواْ دَخَلْتُم بِهِنَّ فَلاَ جُنَاحَ عَلَيْكُمْ وَحَلاَئِلُ أَبْنَائِكُمُ الَّذِينَ مِنْ أَصْلاَبِكُمْ وَأَن تَجْمَعُواْ بَيْنَ الأُخْتَيْنِ إَلاَّ مَا قَدْ سَلَفَ إِنَّ اللّهَ كَانَ غَفُورًا رَّحِيمًا
Forbidden for you are (it is Haraam for you to marry) your mothers (and your grandmothers), your daughters (and granddaughters), your sisters (half and full), your father’s (and grandfather’s) sisters, your mother’s (and grandmother’s) sisters, your brother’s daughters (and granddaughters), your sister’s daughters (and granddaughters), your suckling mothers (those women who breastfed you before you turned two years of age. All the daughters, granddaughters, sisters, aunts, mother and grandmothers of the suckling mother may also not marry the child she breastfed.), your suckling sisters (those girls who were breastfed by the same woman who breastfed you), your mothers-in-law and those step-daughters of yours (girls who are born to your wife from another man) who are in your care and with whose mothers you have had intercourse (with whose mothers your marriage has been consummated). If you have not had intercourse with them (you have not consummated your marriage with the mothers of these step-daughters), then there is no sin on you (should you marry their daughters after divorcing the mothers). (Also forbidden for you are) The wives of your sons who were born of you (the wives of your biological sons. However, you may marry the ex-wives of your adopted sons.), and (also forbidden is) that you join (that you marry) two sisters (real sisters, half sisters or suckling sisters), except what has passed (you will not be sinful for what you did before becoming a Muslim) (In a like manner, a man may not have a woman and her aunt in his marriage at the same time. He may only have one in his marriage at a time). Without doubt Allaah is Most Forgiving, Most Merciful.
From the above verse it is understood that a son-in-law can never marry his mother-in-law and therefore does not have to observe Purdah from her. He serves as a Mahram to his mother-in-law. However, there are cases where mother-in-laws were impregnated by son-in-laws. Many a times, this is the result of the mother-in-laws and son-in-laws enjoying a totally informal relationship, or mother-in-laws not dressing in a suitable manner in front the son-in-laws, etc. Although Shariah has made the son-in-law a Mahram to his mother-in-law, then too modesty, shame, Hayaa, respect, between the two is imperative. When things become too informal in any relationship and the limits are exceeded, then we encounter major problems which are sometimes disastrous.
In short, the son-in-law and mother-in-law can converse with each other without any need to observe Purda but there are limits that should not be exceeded (i.e. they should not become too close and informal). If the mother-in-law is quite young then extra caution should be exercised in the sense that conversations should be kept minimal as there is a greater fear of Fitna, and there should always be a third person around so that the two are never left in seclusion. Yes, if the mother-in-law is so old that generally men don’t desire women of such an age, then there is more leniency, as the fear of getting involved in Fitna is not feared as with a younger mother-in-law.
Regarding the father-in-law and daughter-in-law relationship, just as the mother-in-law may converse with her son-in-law, the father-in-law is permitted to converse with his daughter-in-law as well. However, the same advice that applies to the mother-in-law as explained above will apply to the father-in-law as well. The father-in-law is no exception to the rule, as there are many cases where father-in-laws get involved with daughter-in-laws as well.
In conclusion, caution should be exercised on both sides.