The fact that Eward had been horrible to his sisters made the revenge that Lysander was going to seek upon him all that more meaningful. Mistreat Lysander, well that's bad, but mistreat his family and it's a whole new ball game. Lysander looked down at Edith, biting his upper lip as he thought of whether or not to let Edith completely in on the plan. It was probably best for her that she not know all the details. Plus, he knew she was just as coniving as Eward and may change her mind and tell him.
"I'm not completely sure yet. I do not want him to die, because I didn't die. I just want him to learn his lesson. I won't take over the estate right away, I'm going to let him think everything is fine between us and that I know nothing," it seemed like a good enough explaination, and hopefully she bought that it was all he had planned out so far.