Who Are the Kindred With Whom Ties of Kinship Must Be Upheld?
Question

Allah, may He be Exalted, and His Messenger (peace and blessings of Allah be upon him) have enjoined the upholding of the ties of kinship. 
My question is: 
Who are the kindred with whom we must uphold the ties of kinship? Are they on the father`s side, the mother`s side or the wife`s side?
Praise be to Allah.Who are the kindred with whom ties of kinship must be upheld?
The scholars differed as to the definition of the kindred with whom ties of kinship must be upheld . There are three views: 

That it means one`s Mahrams (those whom one is permanently forbidden to marry)
That it means those who will inherit from you
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 and the mother`s side. 
As for the wife`s relatives, they are not kindred for the husband, and the husband`s relatives are not kindred for the wife . 
Shaykh `Abd Al-`Aziz ibn Baz (may Allah have mercy on him) was asked:  
Who are one`s kindred and relatives? Some people say that the wife`s relatives are not kindred. 
He replied: 
“The kindred are the blood relatives on your mother`s and father`s sides. They are the ones meant in the verses in Surat Al-Anfal and Surat Al-Ahzab in which Allah says (interpretation of the meaning): 
“But kindred by blood are nearer to one another (regarding inheritance) in the decree ordained by Allah.” [Al-Anfal 8:75; Al-Ahzab 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 an authentic report from the Prophet (peace and blessings of Allah be upon him) that someone asked him: “Whom should I honour, O Messenger of Allah?” 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 Muslim. And there are many similar Hadiths) 
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 Allah is the Source of strength." (Fatawa 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.
How to maintain your ties of kinship in Islam
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. 
An-Nawawi (may Allah 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." (Sharh Muslim, 2/201)
Shaykh Muhammad Salih Al-`Uthaymin (may Allah have mercy on him) said: 
“Upholding the ties of kinship depends on what is customary among people, because it is not defined in the Quran and Sunnah in precise terms of type or amount. The Prophet (peace and blessings of Allah 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 Riyad As-Salihin, 5/215) 
And Allah knows best.