My guess is that Sharon refused to be a witch, this recent chapter made us see her and someone that resembled our dear black duke, possibly his father.
I think Sharon not wanting to be a witch for being close to him made someone close to her angry, and then that person cursed the duke as the son of who lead her astray.
There are many holes in my theory like, if Sharon loved the duke father who is Alice dad? Is possible they are siblings, is the no touch rule a cruel joke so that when they discover the truth at least "nothing" happened?
But all this story is weird, why cursing the first son and not all the family or the father, our duke was a innocent without sin after all, was the witch a stalker or obsessed with him and wanted him to stay "pure"until she could cliam it?