do your folks discipine her for things like homework? you know... like, she can't go out 'til homework is done, withholding treats, lifts to her mates etc. Not sure if you are US based (I'm here in the UK) and the UK disciplining is a little different. If she doesn't do her homework, would the teachers not make her do it at school... or could your folks go to the school and say if homework is not handed in, that she misses break time. She's pushing boundaries for whatever reasons. Stamping her feet.
Same with talking back...
Thing is... it's up to Mom and Dad to be the grown ups and enforce the "family " rules - could you maybe suggest a family meeting - maybe once a week... I dunno if that is realistic.... hopefully others will come on and advise other stuff (it's 1am here I gotta climb up to bed now). Theres a great couple of gals on here who I've sent messages too to see if they can post tomorrow.... so please do check back. OK.
Does she throw hissy fits and scream and shout... or is she back chatting and just will not respond to your parents.
Have you tried sitting down with her, maybe in her space - like her room - and having a bro-sis chat? Would that be possible. I guess it depends how you get on. Can you advise her of how you acted at her age?
This must be very difficult for you. But you are a 19 year old lad.. and tho you can back your parents up when kid sis is playing up... you need to let your parents enforce discipline within the home. Family rules are family rules... don't like that word rules.... but you know what I mean. Everyone has to live under the same roof... and no matter the age... at 10 she does know the difference between right and wrong.
Peer pressure, school, friends, hormones, maturing... it all plays a part too... the family dynamics, how mom and dad act.... all sorts of triggers.
Your folks need to handle this. But you can try and talk to her, til she will listen, maybe take her out for icecream or something (maybe she's too old for that - but I would think she probably has crushes on her big bro's friends) - be her friend and her big brother... and let your parents sort out her attitude. There may be reasons you aren't aware of. Keep calm with her, don't belittle her, try and talk. But her behaviour is not your responsibility love. You need to be living your life. Talk to your parents too... if you are all living together - they need to get some boundaries put in place.... for your kid sisters sake, as well as the rest of the family. They have to make the boundaries clear of what is acceptable and unacceptable behaviour. Talking back.... is not one. Homework... if it's not done... why.... does she have any learning difficulties... peers not doing homework either. What does the school do to help this situation? Sorry... lots of questions.
Will be back tomorrow.... keep writing if you want to and that will bump your thread up.... on Sunday, a lot of people wont be posting.... but tomorrow there will be more of us "parent folk" around.... and some older brothers etc who may be able to chip in.
Chin up fella. Like I say, good on ya for wanting to take the time to help your family. Very nice personality trait there. Be proud of you.