Kids have a tendency to listen to a man. It's really infuriating.

Red flag

Maybe she listens better to someone who sounds authoritative. Were you politely asking her to go to bed or actually telling her?

who is she directly related to? it could be that she is pushing back on you if she grew up with you Prior to your SO. or that you don't sound authoritative enough. Guys tend to have a deeper rougher voice so it is easier, same with animal's we had 2 dogs that would not listen to my mother or sister or my high pitched brother... And... if you are not blood related but he is, then she could be pushing boundaries.

Yikes- I hope you don’t have kids with that jerk. Not before some major anger management and parenting courses, geez. You don’t speak to children like that. If it’s not okay to say (or scream) to adults, it’s sure as hell not okay to say to kids. That kind of behavior is a giant red flag.