In the Name of Allah, The Most Gracious, Most Merciful
The term, "social solidarity" in modern terminology, has an equivalent in our great religion (Islam), namely "the kinship ties."
Al-arham are one’s relatives in general. Some have restricted al-arham to be the father's or the mother's relatives, but I think that they are the relatives in general. Hence, if you keep kinship ties with your relatives, their hearts would be filled with love towards you, and as a matter of fact, meeting with them frequently refreshes the kinship ties.
Accordingly, when someone keeps in touch with his relatives, there would be compassion and cooperation among them, in the sense that he strong would help the weak and the rich would support the poor financially.
Dr. Umar Abdul Kafi:
“Kinship ties” means what we call nowadays “social solidarity”, but is it enough to show that we keep in touch with relatives, or should keeping ties with relatives be done with sincere intention?
By Allah, as you have just said; this virtue (keeping ties with kinship) has been distorted. It is performed by putting a visiting card on relatives' house door. One would go to visit his relatives in Eid, but he would be happy to find out that they are out, so his would put a card with his name at the door thinking that he has done his duty. However, by doing so, he does not achieve the purpose of keeping kinship ties.
Keeping kinship ties means keeping in touch with one's relatives, in being empathetic and compassionate towards them and in being cooperative with them. Through kinship ties the strong man helps the weak one, and the educated person persuades his brother to educate his children in order to acquire high degrees.
Some people are poor while others are rich, some are strong while others are weak, but kinship ties narrow the gaps of social inequality down.