There have been quite a few animated entertainment series like that,the most recent was two exes becoming step-siblings.
There's also the one where the fella despairs for his un-obtainable teacher,gets his Deus Vult on with a girl he just met that day due to mutual depressive feelings,followed by him finding out that his teacher is now his elder step-sister and her sister,the lass whose ass was in his hands not too long ago,is now his younger step-sister.
Of course,we have quite a few series where the actual blood related sister gets it on like Donkey Kong with the brother.
Also,yeah,she's definitely feelings all the emotions,she just needs to grasp what they mean,unless she already has and is simply enjoying all to the fullest.