If she's treated like family and under the same father, of course she'll be considered MC's stepsister. They even talk about "not allowed to do that" during their first scene.
The path chosen by the devs is the best one, tbh. On one hand, she's not blood-related to the MC or his father, so they can have fun without getting problems with the no-incest crowd; on the other hand, she's been living with them for so long and has a natural chemistry with MC that the incest crowd can enjoy it 'as if it was incest anyway'.
It certainly feels more organic than those "hey, this is your new stepsister! Oh no, they're already fugging!"