The specifics of your question are a little hard to understand and I haven't played this yet, but step-family is legally still incest.
In what country? Incest is only sex between blood relatives or, in some jurisdictions, legally adopted ones. Step-family is neither and, thus, not incest. Sex between second cousins is still incest but the separation is enough that, in many countries, second cousins can actually get married. Best rule of thumb is that prohibited incest is any degree of lineal attachment, (grandparent, parent, sibling, child, grandchild,) and 1 or sometimes 2 degrees of lateral attachment, (aunt/uncle, niece/nephew, first cousin, sometimes second cousin).
In many places, sex between a step-parent and their step-child is illegal as long as the child is of the age of consent but not yet of the age of majority and in cases where the adult child still resides in the same home with the step-parent, not because it's considered incest but because the step-parent is in a "position of authority," just like a teacher who has a consensual relationship with an of-age student but still goes to prison for "abuse of authority".