It seems strange Sharon would be behind the curse. If she was close to the mother, why would he curse the innocent child, especially when it looked like Alice and Duke Junior were getting close? Even if there's a hole large enough for Alice to pass in the curse, what good is it when Alice isn't aware of it? If the current ruling, sickly duke did something to Sharon and this was a petty revenge, it would still be strange Sharon targeted Duke Junior, endangering her own daughter as well in the process. We know the skull face with, Dulles, is also somehow connected to all this, as she wanted Sharon's diary burned. Perhaps she was merely a merchant or an advisor, who knows, or maybe her role was bigger.
Whatever the case, the cursing witch's words to Duke Junior sounded sort of personal, so either it really was personal (be it Sharon or someone else) or she was kind of crazy if it was merely a bought curse.