@lonelydragon That is exaclty what I am talking about. Maybe they think your sister is a lost cause when it comes to this type of thing. Your parents might feel like she is going to do whatever she wants and it is not worth their time to have a big fight, but they might feel they can influence you. It may feel like they are favoring her, but actually they are showing more confidence in your judgement.
Or, I could be completely wrong and like many older children you might be the one who engages in conversation and negotations with your parents more than younger siblings. Many times the oldest child is very close to the father, being almost a substitute wife (not incest, I don’t mean anything awful like that) but while the mom is occupied with younger siblings the father many times pays his attention to the eldest and they develop a strong bond. Anyway, the older child sometimes becomes much more responsible and adultlike at a young age, because dads tend to treat children as little adults (I am making a lot of generalization here, not true in all families by any means) later it all works itself out when everyone is adults.