So, it's implied that Daleth's older sister simply struck down Sharon with a spell leaving her in a coma that made her appeared dead. Hm, not quite how I envisioned it. I always thought that Sharon being the favorite servant in the household got the responsibility to raise the duke and attend him. Also Sharon was a friend of Daleth and naturally Daleth's older sister wanted to destroy everything Daleth liked and decided to target Sharon. This much is has pretty much been said so far. Here comes the puzzling part. I envisioned that Daleth tried to protect Sharon by casting a spell that would stop her older sister to strike at her directly, so the sister then decided to turn the young duke that Sharon was responsible for into a living weapon by giving him the death curse. Naturally when Sharon would take the duke back to the manor she would take his hand and ZAP! she would be the first person struck dead by the curse. This is pretty much the only explanation that could justify the older sister cursing the young duke with such a horrible curse. The duke was much to young to have been able to do something to deserve such a curse on his own, he was just convenient tool to be used to strike at the real target.
However here we have Daleth pretty much saying that Rob saw Daleth's older sister strike down Sharon, but if she could target Sharon directly it makes no sense for her to put the curse on the duke in the first place. If it was the curse put on the duke that struck down Sharon, Rob would not have known it was Daleth's sister that was behind the attack. I wonder if there is a plot hole somewhere or there are still more revelations to come. It seems the story is now picking up speed and heading to a conclusion