@renkov
Maybe that's the author's backup plan in case the publisher decides to axe it. For a normal ending you either end on status quo, or you need a relatively long build-up to justify their relationship becoming a little more (mentally) stable.
But if Onee-chan is behind everything and does decide to lock him away, you could end the series with only three or so chapters' warning at any time and still have it be memorable, and funny enough to not be dissatisfying.