This is like saying the diatomic vases include monoatomic gasses because there are single atoms in the diatomic gas molecules. The whole point of the nuclear family is that it is indivisible, but easily divisible from other parts of the family. This is very visible in decisions like "can we move away for work?". In a nuclear family, this decision rests almost entirely on whether both parents agree to it and can find work. In an extended family, the grandparents and aunts and uncles (especially the grand aunts and uncles) will have an important word in the decision as well.