Its not a step sister, she is a half sister. Steps sisters are not blood related. Half sisters are, therefore its proper incest in this case, and is more plausible than those children split between parents stories.
Liking the game so far, though the MC (and his father's) expressions are a little awkward, and as some mentioned, there is not many choices to be made in how the story plays out. But lets see how it develops.
One of the father's lines must be corrected: "She's smart, attractive, and very goal oriented. Woman like Nicole are a dime a dozen son".
A dime a dozen means that are common and not very special. He should say "Woman like Nicole are not ..."