Help with stepkid

I’ve been with my SO for almost 4yrs now. We have a 4 month old. And he has a 7yr old girl with another women that has a disability that caused her to start walking when she was about 5 but she has almost overcome that. She is super smart and advanced for her age. Ever since i got pregnant and recently after having my baby boy, his daughter has had a change in attitude. I’ve heard her say little comments that someone her age wouldn’t even know anything about. Like she’s listened to conversations her mom may have had with other people. But when my SO(her dad) isn’t around, she is really rebellious and doesn’t listen to me. She isn’t potty trained yet so when her dad isn’t here, she takes pull-ups off that are soiled and leaves them all around. But when her dad is here, she buts the pull ups in trash bags and throw them away without anyone saying anything to her. When i ask her to clean up toys or play in her room, she ignores me and does something else... I have talked to him about it but its no change... any help before she gets older and it gets worse??