I was wrong about former emperor being so protective… it was about control and obsessed
It still is that, kinda, or rather how it started. His sister wasn't under his control, she just fell for someone who didn't have a title. The obsession came from grief of that loss. The one he obsessed over also left and died during childbirth, so he's blaming a family from "taking her." in his eyes, as soon as they are out of sight, they perish forever.
The Duke truly loves his wife so much as he had to do it in order to protect her ╥﹏╥