Yeah sure it isn't "incest" which is the crime of doing said thing. But the mental and moral issues of dating someone you live with is debatable to person to person, you are at the age of your hormones getting the best of you and you under the same roof, there is no seperation. This is why step-sibling relationships are icky for some and it doesn't bother others.
Edit: I would also like to add that at least these two didn't grow up together for a long period of time. So being attracted to each other is pretty normal, since they didn't develop the sibling bond which would usually sway romantic feelings.