i remember some animes like that and some other type like guy and girl loves each other but their single parents gets married out of nowhere. after that they r thinking that being steps without any blood relation is a no go so breaking up or running away from family.
she really loves him a lot, i wonder when will she realize her emotions. ''dont go my family will adopt you'' or ''son ive heard the story, ill take you to your new home in a jiffy'' were so sweet though. i also tought that she may cry out of relief.
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.