I don’t think she ordered Jeyne to sleep with Robb, because A. the odds of Jeyne agreeing to do that, even if it was necessary to save their family, were really low, and B. her modus operandi is usually to lie to her daughter and use drugs instead.
At the very least, I think Sybell ordered Jeyne to care for Robb, thinking that caring for a handsome conqueror king might cause her to fall prey to the Florence Nightingale Effect. But given that Sybell’s grandmother was Maggy the Frog, and that Maggy may well have taught her how to make “cures and love potions and the like,” along with the recipe for moon tea, it’’s quite possible she drugged either or both of Robb and Jeyne to “make certainty doubly sure.”
And I’m very certain that she did so after negotiating her deal with Tywin, which is incredibly cold-blooded. While some pretty awful things are going to happen in the Prologue of TWOW, I hope her death at Lady Stoneheart’s hands is the silver lining.