Pathetic male teacher, no matter how we don't like him or hate him, still has his role. LOL, he was used as a trigger of topics and blackmailing or something along those lines... If the story between the two charas still develop, I can see that teacher interrupting their sweet little time... Mom seems to be a very carefree person so I guess she won't hinder the two's moments... Who knows, this is manga we're talking about... Anything's possible, for example, like Ryuunosuke said, using the "deep-in-thought-excuse-in-order-to-trip-and-trigger-a-flag-event" kind of thing... But still, Onee-san lusting over Imouto-san is definitely a way to quickly pass my exams... They both got a perfect grade from me... Mostly the bath scene... heheheh~