After a few years, father and son marry another woman with a daughter. At the moment of marriage, the two children become step-siblings, so they are neither siblings nor half-siblings.
The children could marry even during the married period of the parents, since there is no blood relationship, so there can be no incest whatsoever.
The children could marry even during the married period of the parents, since there is no blood relationship, so there can be no incest whatsoever.