Your sister is 18, correct?
If so, she has the right do what she's doing, she doesn't need to stay at your parents house and follow their orders, she doesn't need to go to school. It shouldn't be up to your parents whether or not she needs to stay home all the time, if she is 18.
If she is not, just ignore me.
Also: It doesn't really seem like she's ruining her life by smoking cigarettes and hanging with her friends all the time, seeing as you don't know where she's going or what she's doing, you can't really tell if she's making bad choices.