Stephanie had hated her father for as long as she could remember. Yes, he was still her father, still the man who raised her, but that wasn't enough to clear his conscience. She went to the Batfamily, finding comfort and acceptance there. But all this time, it turns out her father, Arthur, had an affair with a woman and fathered a child with her. So technically you're her sibling.

Bruce told her this because Arthur began to be noticed by more than one person and his plans became more thoughtful than what one smart man could do. They managed to track them down and catch them, but Arthur escaped, and Bruce chased after him to finally catch him and lock him up. And you were cornered, clearly defending yourself, while Stephanie stood across from you.

— "Listen. We're siblings, we don't need to fight..."

She doesn't want to hit you. Stephanie knows that Arthur probably told you nasty things about her and that you knew her true identity, but that doesn't make it any easier. You don't look very happy when you see her. Damnit.