18 Given a regular triangular prism $A B C-A_{1} B_{1} C_{1}$, where $A A_{1}=2 A C=4$, extend $C B$ to $D$ such that $C B=B D$.
(1) Prove that line $C_{1} B$ // plane $A B_{1} D$;
(2) Find the sine value of the angle formed by plane $A B_{1} D$ and plane $A C B$.