I'll dispute that the concept of a nuclear family is a recent capitalist invention in the first place. There have been families long before capitalism came into being. A nuclear family (father, mother, children) is described in a variety of ancient documents and legal codes--Bible (Joseph, Mary, and baby Jesus being one of the more famous ones), Roman and Greek documents, etc. The nuclear family has existed for thousands of years.
The reason that this model is the preferred one is that it one that appears to work the best. Many studies show that children are more stable, happier, and have a much lower incidence of poverty when they grow up in a two-parent household. Do children grow up well in other households? Sure, many do, but they may not have the same advantages they might have had in a 2-parent household. Growing up in poverty does not afford the same opportunities. No family is perfect, but the traditional nuclear model at its best appears to work in a superior way to other family models at their best.