Hi.
The term "incest" usually refers to close blood relatives such as father and daughter, mother and son, or brother and sister having sexual relations with each other, though sex between stepsiblings is not generally welcome either.
There's a whole body of theory around why the act is so frowned upon. Some believe incest is discouraged to prevent genetic defects caused by inbreeding. Others believe it has more to do with keeping the order in close familial units, and preventing tension and competition from seeping into the main group in which people rely on each other to take care of them.
That is certainly more the issue in your case, since you are not related by blood to your stepbrother. Your mother and stepfather (and friends) would likely not look favorably upon the situation, and may even seek to separate you two in some way if you are found to be involved with each other sexually.
Imagine, however, the confusion that might spring up between you and your stepbrother in such a case. How do you relate to him at home--as sister or a girlfriend? And what if the relationship hits the rocks? You would be forced to live with your stepbrother in the same house, suffering in silence, surrounded by your parents who might eventually catch on anyway.
The topic is so loaded, especially since you are both young--the prohibition against teen sex will make the situation even more explosive. For these reasons, I'd recommend that you and your stepbrother take some space away from each other. Date other people and try to keep this a nonsexual relationship. When you get older and are no longer in your parents' home, if you still feel this way, the situation might be more workable.
Take care.
