Is it normal for a child to act this way?

This all stems from shit parenting and mother being a total bitch, but this behavior is fucking rediculous. I swear my boyfriend's 10 yo sister is the most ill-behaved child ive ever seen. The mom is lazy and doesnt want to deal with her kid, so she basically put all responsibility on my bf, making him pretty much be like a full time dad, only if he diciplined her, he got yelled at by mom.

He tries to help her with her homework, (mom gets frustrated quickly and wont do it) she litterally screams, squeels at him to do it for her. Acts like she cant read sometimes but obviously can. She'll stretch out an assignment that would normally take 10 minutes, to about an hour because of her tantrums. Eventually he gives up. Once he was busy so she came to me and asked (demanded) that i help her with her math homework. Knowing what id go through, i told her nicely to ask her mom to help. She goes into her moms room and said i was mean and wouldnt help. The cunt has the nerve to bitch me out 'why didnt you help! RantRantRant' and told the kid to give me the silent treatment. For not helping HER kid.

She acts like she cant make herself basic meals when the mom doesnt cook. Yet she does all the time when hes not there. Therefore she bugs her brother to drop everything and make her something. Which most of the time he refused, to (try to) teach her to self reliance. But again shell have a tantrum like a 3 year old and go complain to mom and hed get yelled at for not making her something.

Before he moved in with me, the mom would expect him to give her the majority of the money he earned at his mechanic job. Mostly to buy the kid toys she doesnt deserve nor even play with. She even made him pay for a hotel+pizza birthday party for the kid. Yet they both still expected him to get a gift.

The mom and sister (mostly sister) even bothered him to do for them while he was doing privite things. He would be in the shower, they would beat on the door saying the need in there, though he would ask everyone if they need to use the bathroom before he showered. He would be trying to sleep, the sister would beat on the door 'john! You going to sleep!? You sleepin!? What are you doin?!'. It got to a point where he just stopped responding and kept his door locked.

The sister demands control of the tv always. If hes trying to watch something, she'll either grab the remote or stand in front of the tv and until he gives it up.

Once he was trying to tweeze a tiny splinter from her foot, it was like the devil was coming out of this kid. Im not even religious, but that was like the exorcist.

He finally got tired of it all and moved in with me. Hes now much happier. Currently we're ignoring the moms calls.

This is the most rage inducing kid. Id never felt a stronger urge to punch a person hard in the teeth until i met the mother and kid(of course i wouldn't.. The kid that is)
is it normal to be such a shit parent and for a child to act this way?