nope! their relation was never established in canon, only that they were close. however, during the explanation, mark said that he had the idea of damien and celine being siblings. so, people jumped on board with that idea, myself included. i think it just makes it more interesting, y’know?
like if damien and celine are related, did he know that celine and william were having an affair? was he closer with mark than with the others? so many questions lol