The legality of a stepson marrying a stepdaughter would depend on the laws and regulations of the specific jurisdiction where the individuals reside. In many places, marriage between step-siblings (those who share a parent due to their parent's remarriage) is considered legally acceptable because they are not biologically related. However, some jurisdictions may have specific laws or cultural norms that restrict or prohibit such marriages, so it is essential to research the laws in the specific area.
It's crucial to recognize that marriage laws can vary widely around the world, and what might be acceptable in one place could be considered taboo or illegal in another. If a stepson and stepdaughter are considering marriage, they should consult with legal experts or seek advice from local authorities to understand the regulations governing such unions in their specific region.