beta
Per my understanding they aren’t half siblings, or genetic siblings, technically they were step siblings. TECHNICALLY
Blondie at one point says that her mother brought her to the house and that the duke isn’t her father. The duke didn’t know the chained up boy was his son but the red haired bitch did know. So on a technical legal stance, because the red haired woman was married to the duke, that made blondie and black haired boy step siblings.
Since the mother is no longer married to the duke, blondie and black haired boy are not really step siblings anymore.