There's some good children's books that explain autism in a child's terms. A good place to start.

In reference to younger siblings picking up traits, they absolutely do. We learn by modeling what we see and a younger sibling looks to older siblings to find out how to behave, interact, play, talk, etc.
My sons are 14 months apart in age. Although they now, many years later, both have a diagnosis of Autism, the doctors at the Autism Center concluded at the time of my older sons evaluation his brother was showing no signs at that time. Behaviors that we grew concern for later were determined to be Mirroring of what he was observing with his brother.