As far as the lie goes, if she joins the Sith they at least won’t be dead, and she won’t hurt them because she’ll be in control of her powers. That’s a start. And as long as they’re alive, maybe she can convince them that this is the only way.
In that way it is similar to Anakin in ROTS. He knows the Sith are problematic, but he hopes he can convince Padmé it is the only way they can be together, and make things the way they want them to be.