I doubt you can change the way your parents are handling your sister. They have their idea about how to do this. In many regards grownups are more stubborn than kids.
Kids with aspergers are sensitive to impression and input, so they need to get very selective. They have a strong need to find an identity to set themselves into. It's all important to gain their trust and affection. If a kid with aspergers get a strong and trusting relationship to a good individual, they will turn out just fine.
The problem with these kids is that parents are so detached from their kids these days, and only have time for occational bonding, while they still have to spew verbal guidance over them on a regular basis. A kid with aspergers won't listen to words unless the one who talks to them is someone they have deep trust and respect for. But if they do, they will listen ever more closely. Either you have their full trust and respect, or you have none.