No reaction

I'm wondering why Mrs. Brisby had no reaction to Sullivan's remark of "Jonathan Brisby is dead!" in the council chamber. She showed no shock or sorrow, at least not on the outside. I expect she assumed he was dead, but didn't know for sure before then. Unless she was too overwhelmed by everything going on to really take his statement in.

I never noticed that before...it is very strange she doesn't react.

I think she didn't hear it over all the yelling..

IMHO, she knew it already... There are lot of stories about the mysterious link between those who really love each other... She knew it from the very evening Johnatan didn't came home... Besides, in the world of field mice, surrounded by hundreds of threats there is no such things as "gone missing"... Beloved and loving father and husband, Johnatan couldn't just leave his family for such a long time (the whole year!) without a reason! Only death could separate them - and Mrs Brisby knew it...
And remember the very beginning of the movie, meaning, Nicodemus' monologue? "Johnathan, wherever you are, your thoughts must comfort her tonight. She'll be waiting, and you will not return..." Taking in concern the mystical part of the movie I think somehow his soul reached Mrs Brisby and thus she was aware of his fate... Maybe subconsciously, but still, she knew it for sure and somehow managed to keep living - for the children - casting away, hiding her sorrow deep inside... Sometimes it's time to leave the past behind and she did it - resigned to her husband's death.

I think she may have known since before the movie started. When she meets Mr. Ages in the beginning, he says, "I am sorry about your husband's death."
So she was probably already aware of that fact. But her apparent lack of reaction with the way Sullivan said it does leave a question.