The father did not remarry if I remember correctly. With the incest patch. Ashley's daughter is indeed the biological daughter of the mc's father. => Half sibling, not step sibling. Same father, two different mother.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.