Is Sex Between Step-Siblings Wrong?

I was asked an interesting question the other day:

"Is it considered incest if adult step-siblings have sex?"

Before answering I had to first set aside my personal moral stance and think through the question. I personally know two couples who are adult step-siblings and are also married to each other. So, obviously the legal answer to the question is no, it isn't considered incest in the eyes of the law.

In the two cases that I mentioned above, all 4 step-siblings met as adults when their parents began dating each other. They didn't grow up together and they never had a sibling-type relationship.

In the case of younger, underage kids I think the moral dilemma is evident. In my opinion, if the kids were in a family setting and were treated like siblings then having an intimate relationship, even as adults, should be out of the question. The judgment call cannot be made on blood-ties alone. Otherwise adopted siblings and foster siblings could be considered potential sexual partners.

What is your opinion?