Step-sibling does not = half-sibling. A Step-sibling is NO blood relation to you whatsoever. It might be that your parent (widowed or divorced)married someone who came to the new marriage with a child from a previous marriage. That child is not assur to marry, and I know frum people who have in fact married their step-siblings.
If however, your parent marries again and sires/bears a child with the new spouse, THAT child though a half-sibling, is certainly considered your full sibling in regards to erva, and would therefore be assur to marry. Some people might have a special sensitivity in the case where the step-siblings have been raised as siblings from a very young age, so that they are emotionally connected as brother/sister, even if they are not biologically related to each other. That is another issue, but still not technically assur should they want to marry.