Be rude, thats what I do to my step brother that has a thing for me, I avoid him, cut him of when he tries to strike up a conversation, and tell him he is gross.

You say you're not unhappy about it, so I'm guessing maybe you're at least a small bit interested? If you're just flattered, I would say kick your ego in the rear and cut it out. If you can see that you're enjoying the attention, but that it's not a healthy thing, you are only making yourself unhappy in the long-term, and in the short-term, you're both leading him on and also potentially creeping out your family.

If you are truly interested, there is nothing illegal or incestuous about pursuing a relationship, but you should be ready for some serious flak from family, friends, and acquaintances. This man is not your brother, and since you didn't grow up with each other, you might not think about each other as really being family. You could, conceivably, have a perfectly normal healthy relationship. As they say, all's fair in love and war. You certainly wouldn't be the only married couple in your situation. Just don't expect your family to necessarily agree with your choices. (And can you imagine what family dinners would be like if you dated and broke up? There's an uncomfortable situation...)

If you are going to pursue it, I would say be absolutely sure he's really flirting with you, and not teasing you in a sibling kind of way. Not only could you really embarrass yourself, but if he does think of you as his sister, that's the kind of awkward that lasts and lasts.

If you're not really, really interested, I would say save yourself what could become the biggest nightmare of your life and expand your circle of options. Shop around a little and busy yourself with something or someone else. Consider that you might be seeking out some drama, frankly, out of boredom and pursue a hobby. Meet some other guys and go on some first dates. Expand your options before you decide this is worth anything.

If you do decide to back away from his attentions, let him know you find all his "kidding" a little annoying. That should spare his ego if he was flirting, and allow the whole situation to just fade out.

don't talk to him,try to avoid him otherwise it will be problem for you to get rid of him.if you give no attention towards him and always try to keep distance,telling him brother or bro not by his name and if he is with you make sure that others members are also there.this things will help you.

Even though under the law it is not incestuous, it would obviously cause several unfortunate conflicts and situations to arise within your family. This is perhaps what you have to consider the most.

Before proceeding in any direction with this, you must determine what in fact is actually happening. You must open communication on the subject, in confidence with your step-brother.

If you are both interested, you should both pause and reflect on the point I made above, is it worth it? Is a carnal fling worth all the hassle, turmoil and confusion that this will cause in the family.

If the answer is yes, and if you proceed with it, my suggestion would be to also lay out a plan to address the issue with the key family members. It is best they hear of this from you, rather than to discover it unwittingly.

Are you interested in him ? Yes, then go ahead.