If the equation of the pair of straight lines passing through the point 1 1 and one of these making ...........

If the equation of the pair of straight lines passing through the point (1, 1), and one of these making an angle $\theta$ with the positive direction of x-axis and the other making the same angle with the positive direction of y-axis is $x^{2}-(a+2)xy-y^{2}+a(x+y-1)=0,a\neq =2$ then the value of sin 2 $\theta$ is