Responses
It was stated before in the early chapters that he was illegitimate , his half sister was was the nanny/ maid ( the one now married and with a baby Marie) of Florentia always wanted to have a relationship with him as siblings but he didn’t want to since he couldn’t explicitly say that he was her brother, I think they said he was a “ cousin” brought to become an heir because the father didn’t have male heirs.
i was actually confused when he was like "how did she know we were related??" because i genuinely thought that them being father and son was so extremely obvious that it didn't even have to b stated