My 8 year old cousin is disrespectful

Quick backstory, my cousin and his 8 year old daughter live with my fiancé and I and our 3 year old son. They’ve lived with us since she was about 3-4.

All things she’s done in her younger years aside, she’s still constantly disrespecting everyone in the house. My aunt and other cousin have watched her every day since they’ve lived with us, but they’re not the best of people. I don’t talk to them or have any relationship with them. Why my cousin chooses to send his daughter down there is beyond me.

Anyways, she’s extremely rude, stares at you with a blank face when she’s in trouble for something she did (ex. Hit my kid, get slime all over the bathroom and everyone’s toothbrush and just leaves it, constantly uses my face wash/shampoo/conditioner to make weird concoctions after I’ve asked her not to, hides TV remote on purpose.. the list goes on.) she one time in the middle of the night took a knife to the kitchen countertops because “she was bored”. We had just put on cheap peel and stick paper over the countertops and she destroyed them.

I don’t want to overstep my boundaries as she’s not my kid, but I’m honestly at a loss for what to do/how to handle her.

She was standing on my sons sit n spin tonight and asked her to get off because she’s too big and will break it. She just stared at me and continued standing on it. I asked her again, and she stared then with the snarkiest attitude she goes “well maybe if you said PLEASE I would listen” I then told her to go upstairs and she’s not allowed to play with my son until further notice.

She goes to leave and says “I’m gonna tell my uncle on you” I was like uncle who? She says uncle bob. (Names changed, but that’s where she goes during the day) and I was like okay? And what’s he gonna do? She says he’s gonna snap on you. I said go for it, tell uncle bob I don’t care. She goes just wait and see then walks away.

Like what??? Is this NORMAL? Because I know I would NEVER talk to an adult like that when I was a kid. Although I know everyone is different, but I just honestly don’t know how to handle this situation appropriately. I tell her dad about these things and she “gets punished” (whatever that entails, probably nothing from what it seems) and she continues doing the same stuff every day.