Which 'sister' will he end up with, notwithstanding the possibility of even more 'sisters' appearing? If he ends on the step-sister route he will still have to wait 3 more years to stay on the right side of law for anything too risque.
Should this be considered a harem or does the little sister read all the siscon things because she thinks she wants to be just a sister? Obviously there will be a little cousin or childhood friend too