I was really dissapointed with this ending. Then I thought that it's open enough for a head canon were he got together with the senpai instead while she was away.
@Lioslin remember the brother has perfect defense. My guess is that while the owl can't harm him, they can't attack it either. It probably keeps killing/destroying all of the sister's undead minions and that's why she's tired (from using so much magic).
I thought it was a good ending. There was never tension on this series, just suddenly ending would had felt out of no-where (looking at you Between Smooth and Rough) so the author introduced a little bit near the end, but like the rest of the series reality is much simpler.