I know a 15 year old with Autism who recently does not like anything about his 12 year old sister. when she walks in the room, he blows the air to get rid of her germs and will not be close to her at all. He will not talk to her, will not use the same bathroom as she does, does not like the sound of her voice, etc. He pretty much does not like her at all. He is fine with his 10 year old brother and his parents and even anyone else that he may come into contact with. this, of course, is very upsetting to his sister. She wants to look up to her older brother so much but it is so hard for her right now. you can imagine how hard this is for her and the rest of the family. Does anyone have any ideas as to why this troubles him so much? it has only surfaced within the last 2 years or so.

most autistics have a social isolation problem, and tend to be isolated from the world. unfotunately, autistics do not know much difference between bullies and "good" people, so he might have accidentally mistaken his sister as a bully.

Autism or adolescence?
Only based on this fact, seems adolescence rather than autism.
Quite natural, take it easy.