According to Marxist thinking, the nuclear family serves an important role in capitalism. It is a bourgeois institution, or at least was in the nineteenth and early twentieth century. The family was a place where people could be educated and indoctrinated into the values of capitalism. It also reflected a gendered division of labor, since women took care of household duties and men served as "breadwinners." In Western societies, it was also the mechanism by which the bourgeoisie transmitted their wealth from one generation to the next. Young men inherited property, money, and status from their fathers. In The Communist Manifesto, Marx and Friedrich Engels defended the radical position that the family ought to be abolished as an institution:
On what foundation is the present family, the bourgeois family, based? On capital, on private gain. In its completely developed form, this family exists only among the bourgeoisie.
They did not mean that the relationship between children and parents, nor between siblings, should be abolished. Rather, they argued that the concept of the family as currently understood in Western society was basically superstructural. It functioned as a device that undergirded capitalism. Once capitalism was destroyed, the idea of the family as currently constituted would fall as well.
Really, Marx used the idea of the family to illustrate an important point: What people imagined as "eternal laws of nature and of reason," such as contemporary concepts about the family, were actually historically contingent and superstructural. Societally sanctioned forms of relationships reflected the arrangement of economic forces in the societies where they existed.