Autism and Asperger Syndrome are not that different.
The only real difference between diagnosis Asperger Syndrome and diagnosis of Autism Spectrum Disorder is that the latter has developmental delays. There is no other clear difference outside of that distinguishing factor - thus why Asperger Syndrome was merged with Autism diagnosis, and it's commonly believed that many who have the diagnosis of Asperger Syndrome do so only because doctors preferred this diagnosis to attaching the stigma of Autism to children or scaring parents.
Autistic people are all different and impacted in different ways by their neurology - you have met people with Asperger Syndrome who are different to your sister, who show different characteristics or have different functioning abilities, that doesn't mean that Asperger Syndrome and Autism are hugely different. If you were to take a person with an AS diagnosis and one with an ASD diagnosis, both the same age and with similar characteristics, you would not be able to tell the difference. Your peers seem different to your sister as they don't have developmental delays and have different communication problems (e.g. they've clear social communication issues, but fewer general communication problems).
I'm diagnosed as Autistic as I was non-verbal as a child, along with other characteristics I was very severely affected as a child - it sounds likely I was more severely impacted than your sister when I was her age - by the end of high school I was closer to how you describe the people with Asperger Syndrome you know in high school, and as an adult I can pass for neurotypical. For the record I've a very high IQ and don't have mental or physical developmental delays, also I know many people who have a diagnosis of Asperger Syndrome but who have far more severe problems compared to me despite my ASD diagnosis.
The idea that Autism = 'low-functioning' / Asperger Syndrome = 'high-functioning' is false.
This is where the spectrum comes in, it's not just that the autism spectrum is one straight line with more severely impacted people (e.g. Autism) on one end and less severely impacted people (e.g. Asperger Syndrome) on the other. As well as the fact that how a persons autism presents will typically change dramatically throughout their lives, but how a person is affected and the severity will differ too - e.g. your sister may have more severe communication issues to those you've known in high school, but for all you know those you've known in high school may have more severe executive dysfunction than your sister.
This is a nice explanation of the spectrum that explains a little more of what I mean:
http://alannarosewhitney.tumblr.com/post/113254781979/shades-of-slander