To be fair to Dio, I personally read that last line of his less as him treating Nina like a possession, and more like. . .
“Don’t just send her away / kick her out of the castle / be like randomly byeeeee~.
If you no longer care for her, or if you decide she’s too much of a risk to keep close, then I will care for her.”
But that’s just how it sounded to me. Could be wrong. 🤷♂️