Yes but why? Why does it have to be her and not the woman he did marry?
The prophets have that retarded cylon thing going on where they just make people do stuff with no seeming understanding of the effect it has.
Because that's who it is. Think of it as a fixed point in time, if that helps. As non-linear beings, for the Prophets both things - meeting Ben and sending Sarah to Joseph so that Ben would be conceived - were happening in the "now."