Nah, I feel like the sister is at fault 😶..
He can't exactly speak against it as in just blurt out their relationship to him and he's aware that he's a important person in her job, which makes it even harder for him to say anything, though he already made advances on his sister by the sound of it, but she apparently never outright rejected him, or told him she's not interested
He can't exactly do much in that situation, even if he were to speak up its not like she would just quit her job and stop interacting with him