I have to wonder why the company ever assigned this employee to this disabled man if they had this knowledge prior to the employee working. Does this mean that you knew of the rape, but not who the victim was or that the employee was related to his victim?
I would say you have a conflict of interest that you now know about and would put that employee on another case. I don't see where any illegal discrimation would come into play, but maybe I am missing something. Do you have a conflict of interest policy? To me the liability if something should happen to either party would rest squarely on the shoulders of the employer. While it may not bother the employee, the facts are not good.
Some states protect employees from non-work-related issues....so you might confer with your company's attorneys to see the extent of your liability since you now know that it is possible there is a larger risk of something going wrong. You didn't state if this employee was male or female, but in my mind, that would not matter. The only salient point is that there is a non-work-related link between the two. And again hopefully you already have some type of conflict of interest policy.