There was a very valid reason why she chose not to go straight to Antonin with information about his brother. As much as Antonin might have claimed he wanted his brother dead, Hermione feared that faced with the actual moment of killing his younger brother, her husband wouldn't be able to do it. She didn't want there to be a single opportunity for mistake. Also, she had a legitimate fear that if Antonin was aware of what his brother did and told Thorfinn before she had a chance, the blond wizard might kill the murderer of his wife before she had a chance to help. That was not an option in her mind. She had to see the light leave Aubin's eyes before she could relax again. Antonin would try to keep her out of it if he could. Maybe he would think that he was "protecting" her. It didn't matter. She wouldn't let him take that experience away from her no matter how many lies she had to tell and secrets she had to keep.