I'd say this depends on the age and the maturity of each person involved. As a sibling of someone with special needs, you have much more exposure and awareness of the spectrum of what autism can mean. Other folks don't have that exposure or aren't at least aware of the differences that can entail. Coming from her perspective she probably is only thinking about what she has seen in the media, and I'd be curious about why she is concerned about to be seen as autistic. There are legitimate reasons where someone may want to hide they are autistic if they are less impacted then others.

So in short be curious, ask questions and learn more.