Even tho it's clear Seiko is still grooming Sei I do believe part of what she said was genuine. She always despised her husband for never standing up for her when she needed it the most. Even going as far as to keep keeping and laugh with them at her expense. So seeing Sei stand up for might have actually moved her tears. Which isn't a good thing for Sei because this only means the grooming will continue, and more boundaries will be broken.
Also notice how dad finally is able to stand up, but for himself, not his son. When Sei needed him most he was off doing his own thing, but now that he sees he is becoming more like her (even appearance wise), he leaves him with the monster not fully grasping that he contributed to this downward spiral with his inaction.
He wasn't there for his wife, or his kid, only himself and HIS family. The one his wife and child aren't apart of.