Line-Plane Angle in an Oblique Triangular Prism

In the triangular prism $ABC\text{-}A_1B_1C_1$, the lateral face $ABB_1A_1$ is perpendicular to the base $ABC$, with $AB=BB_1=2$, $AC=2\sqrt3$ and $\angle B_1BA=60^\circ$. Let $D$ be the midpoint of edge $A_1B_1$, and let $E$ be the point on edge $BC$ with $\overrightarrow{BC}=4\overrightarrow{BE}$. Suppose $DE\perp BC$.

1. Prove that $AC\perp BB_1$. 2. Find the sine of the angle between line $BB_1$ and plane $DEA_1$.