They are going to beat Mister with their stupidity. Mmm, I need to catch up with this series, it has been a while since I ignored it.
Not stupidity. Optimism, or alternatively, naivete. They relentlessly believe that as long as they're a family, they can take it easy, no matter how bad it gets. Mister wants to be torturing shitheads, but no matter how hard he tries to turn these yukkuris into shitheads, they remain unflinchingly nice.