Furthermore, if willfulness is purely inborn (or genetic) wouldn't siblings be more likely to exhibit similar strengths of will, since they have such similar DN

Furthermore, if willfulness is purely inborn (or genetic) wouldn't siblings be more likely to exhibit similar strengths of will, since they have such similar DNA? In a family, nature and nurture are mixed together. The only way to separate the two components are with twin studies where the twins are raised separately.