@Miru No, because they are not blood-related. They're stepsiblings. Ex.: If my mother marries another man and this man had already a daughter from a previous relationship. That girl is going to be my stepsister.
@THE MOST ANCIENT DREAM Well I mean as long as they don't end up together I'm good the problem I originally had was that I was worried they would end up together andeven if they're step siblings it still count as incest
@Miru it is not incest as there are no blood ties between them, incest occurs only between blood related people, like siblings, parent and child, sibling of a parent and nephew/niece, cousins and the likes. Morally it could be close (but still not actual incest) if they grew up thinking of and regarding the other as a true sibling since they were infants, but this is not the case here