Randolf did not expect Aldrian to actually say that he had a solution. In his plan, his main target was to know whether Aldrian was experiencing the same thing as them or not.
He was willing to take the risk of revealing his family's problem, which was practically the same as exposing their weakness to Aldrian, solely because he thought that in return, he would also learn Aldrian's weakness.
That information would be crucial for him because it would mean that the chance to get rid of Aldrian had finally come.
Even if Aldrian later became suspicious that he was an apostle of a god after learning about his family's weakness, it did not matter. As long as he kept denying it and never showed his system, Aldrian would never be certain that he was an apostle.
