It depends partly on whether or not the step-parent legally adopts the child; if they do, then the child is legally indistinguishable from a biological child. It also depends on state law, which varies. Woody Allen famously married his stepdaughter; I’m not sure how he worked out the legality. Social acceptance is another issue entirely, as are the emotional impacts on all parties. In other words, “it’s complicated” means that if someone wants to navigate this path with minimal harm, it will take a lot of work. Someone would need to consult with a wide range of services to do it well: family law, couple and individual counseling, and probably a licensed social worker, all of whom may refer involved parties to additional services.