@djmordred There is a reason nobles do that. If he would have said it was her mother, that is effectively an open declaration of war. The castellos can legally attack him to rescue the mother.
By hinting that he has her mother though, it turns into speculation which means she has to confirm it first if she wants ti take any action.
By declaring the mother dead, it serves 2 purposes, limits the use of the mother in society, and also says, if she dies we dont really care, so dont expect anything grand in negotiation