Seiichi is being dumb with his explanations to the police; he's already become aware that they're trying to establish a motive for Seiko's behavior, and asking him about it, but he's telling them nothing they can use. What he did there and what he told his father are completely opposite in action. There is a slim chance that Seiko may serve little time, if it can be shown that she is crazy, or that she was 'driven' to her actions by some turmoil brought on by her husband's family. The fact that no one actually died is a considerable to her case.