i need help... i have the spawn of satan living with me part time!!!!!!!!!!!!!

why is that my step daughter feels the need to take whats not hers? like she took her brothers flip (with out asking) to use for her own purposes. then her brother out of the kindness of his heart let her use his gamer id for a game and she got him kicked ff because she said in a chat on the game the she "fucking hates this game" first off shes 12 yrs old and using that kinda of language. she needs to be knocked down a peg but her parents refuse to do it. take her damn phone away. its not hard. if she starts crying dont give in. but of course she throws a big fuss and gets her way. why cant parents just stick to punishments that they enforce. instead of giving in when she turns on the waterworks and says things arent fair. it piss me off that she doesnt even care that she caused her brother to get kicked off his game. she only cares about herself. how can i change that? i cant. why cant se just get a good smack in the face. maybe that will knock the whole "im way better tha you are and i dont give a shit how you feel about it" attitude. another exmaple would be she is sitting on the couch and the dogs are scratching at the window screen 10 ft away. instead of saying stop or getdown she coses to ignore them and the knock the screen out and she does nothing. only when i come down the stairs does she go out and get the dogs. i ask her why she did watch the dogs... her response "they arent my dogs. im not responsible for them. im not going to watch them." all i can do is get pissed and walk away. i cant yell at her cuz she tell her mom i called some god awful name and that will start ten kinds of shit. what to do what to do...
now the boys on the other hand are amazing. i have no problems out of them at all. none. they do what they are told to do. they clean up their messes. i dont have to worry about them doing anything bad because they dont. why cant she be more like them. if y'all have any suggestions on my spawn problem please fell free to comment. im not a mom so any help on how to combat this disease would be nice!
thsnks to all and stay cool
E