Praise be to Allah.
The scholars differed as to the definition of the kindred with whom ties of kinship must be upheld. There are three views:
1-That it means one’s mahrams (those whom one is forbidden to marry)
2-That it means those who will inherit from you
3-That it means all blood relatives, whether they are heirs or not.
The correct scholarly view is the third one, that it refers to all blood relatives – not relatives through breastfeeding – on both the father’s side and the mother’s.
As for the wife’s relatives, they are not kindred (arhaam) for the husband, and the husband’s relatives are not kindred for the wife.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him) was asked:
Who are one's kindred and relatives? Some people say that the wife’s relatives are not kindred.
The kindred (arhaam) are the blood relatives on your mother’s and father’s sides. They are the ones meant in the verses in Soorah al-Anfaal and Soorah al-Ahzaab in which Allaah says (interpretation of the meaning):
“But kindred by blood are nearer to one another (regarding inheritance) in the decree ordained by Allaah”
[al-Anfaal 8:57; al-Ahzaab 33:6]
The closest of them are fathers, mothers, grandparents, children and their children, no matter how far the line of descent goes. Then the next closest and the next closest of brothers and their children, paternal uncles and aunts and their children, and maternal uncles and aunts and their children. It is narrated in a saheeh report from the Prophet (peace and blessings of Allaah be upon him) that someone asked him: “Whom should I honour, O Messenger of Allaah?” He said, “Your mother.” He said: “Then whom?” He said: “Your mother.” He said: “Then whom?” He said, “Your mother.” He said: “Then whom?” He said: “Then your father, then the next closest and the next closest.” Narrated by Imam Muslim in his Saheeh. And there are many similar ahaadeeth.
With regard to the wife’s relatives, they are not kindred of her husband, because they are not his relatives, but they are kindred of his children from her. And Allaah is the Source of strength.
Fataawa Islamiyyah (4/195)
The relatives of each spouse are not kindred of the other spouse, but they should still be treated kindly, because that is part of the good treatment of the spouses towards one another, and it is one of the ways of increasing love.
Upholding the ties of kinship may be done in many ways, such as visiting, giving charity, treating them kindly, visiting them when they are sick, enjoining them to do what is good and forbidding them to do what is evil, and so on.
Al-Nawawi (may Allaah have mercy on him) said:
Upholding the ties of kinship means kind treatment of relatives according to the position of each of them. Sometimes it may be by giving money, sometimes by serving them, sometimes by visiting them, or greeting them, and so on. End quote.
Sharh Muslim (2/201).
Shaykh Muhammad al-Saalih al-‘Uthaymeen (may Allaah have mercy on him) said:
Upholding the ties of kinship depends on what is customary among people, because it is not defined in the Qur’aan and Sunnah in precise terms of type or amount. The Prophet (peace and blessings of Allaah be upon him) did not limit it to any specific thing, rather it is general in meaning. Hence reference should be made to what is customary. Whatever is customarily regarded as upholding the ties of kinship is upholding the ties of kinship, and what the people customarily regard as breaking the ties of kinship is breaking the ties of kinship.
Sharh Riyadh al-Saaliheen (5/215)
And Allaah knows best.