I come from a comfortable family economically.
I have a younger brother and a younger sister, and from as young as 6 years old we've been in private schools(as opposed to free government schools-which are very bad educationally, at least in this country).. My brother and I both did all right too good, in school hardly studying, doing homework e.t.c (I'm currently studying in a top UK university).. My sister has always been struggling in school, and for the past 3-4 years on the verge of failing... My parents, being naturally concerned, even went as far as hiring private teachers along with her taking lessons in the private school, but she's always barely passed.
She's really intelligent when you speak to her, very witty in her responses, but always causes trouble in school e.t.c and causes stress to both my parents. My parents have tried everything to get her to change her ways, from threatening to throw her into a government school(where for someone who is as uninterested in education as she is) it's guaranteed that she'll be a nobody in life. They've tried reasoning with her, grounding her, even hitting her at times when she honestly deserved it(we're not a violent family but she really is unbelievable at times).. She lies, she is selfish and unappreciative of the treatment she receives..
She might be grounded but my mum will eventually feel sympathy for her and she'll let her off the hook..
She's supposed to be grounded atm for failing the year(suprise suprise), but she had friends over again.. and I just found out she took the car for a spin with her friends... (legal driving age is 18 here).. I'm really speechless at her actions.. and I had a 4 hour conversation with her and tried to reason with her, contemplating whether I shuold tell my parents or not, but I probably won't because I know what sort of stress it puts on them..
She promised she would change but she always promises and she will only f*** up again.
I don't understand what we can do with her.. Ship her to boarding school or something? My parents could never do something like that.. I mean I'm her brother and I've given up on her I don't know how much more patient my parents can be.. its been years she's just been causing trouble and hasn't cared for anyone but herself.
You can't blame bad parenting either, my brother and I would also cause trouble when we were kids but it was a normal phase we went through just as all kids do..
I feel no sympathy for whatever punishments she receives now and if anything I get annoyed at my parents when they eventually let her off the hook... They just feel sorry for her because she's nearly always grounded.
The second she's "let loose" she'll do something else rebelious and stupid.
Anyone..

Maybe she's just not a school university type. I know it's a bad excuse but I have a little brother who is very witty and smart when he speaks, he just doesn't understand that school is a good thing. He can't grasp the concept of liking school, its not in him. So year to year he just barely passes by and no one does anything to make him correct his error. This year my mother forced him to do his homework and turn it in. If he didn't she would 'punish him' by no video games, or computer, or some other meaningless task or chore. It didn't do much. I feel your aggravation. He is given every method and path for success, its put in his nose and he won't take it. Im not really sure how to fix it, but it sounds to me like your sister either doesn't care at all about anyone who helps her or else she really doesn't know how else to act. Maybe a boarding school, or here in the states we call it a tough love school, maybe one of these would help. And less sympathy from your parents. She caused her punishment, its not like you just don't like her, she messed up, so she needs to be punished. Until that happens, there is no help, no light at the end of the tunnel.
Hope it helps
~Destiny~

Unfortunately some people have to learn the hard way and usually there isn't much you can do about it. Often they do grow up and realize life is about responsibility and you can't keep messing up. Hopefully she will come to realize that soon. I can understand your position because you want to tell your parents but at the same time it can cause more problems. Like if you tell them things they don't know it could break your sisters trust to where she doesn't confide in you. Then she will be out doing who knows what and nobody to keep tabs on her.
What do your parents say from what they do know that is going on?

Oh and if the public schools are anything like they are here she would be a lot worse off because she would be able to get away with a lot more besides academically.