Let $\overline{a},\; \overline{b}$ and $\overline{c}$ be non-zero vectors such that $(\overline{a} \times \overline{b}) \times \overline{c} = \frac{1}{3} |\overline{b}| |\overline{c}| \overline{a}$. If $\theta$ is the acute angle between the vectors $\overline{b}$ and $\overline{c}$, then $\sin \theta$ equals
