My less-than-two-years-younger brother is autistic. Now that he is in his late 20's and enough random people have pointed it out, my parents have very nearly excepted that he might have Aspergers Syndrome. Meanwhile, people who have any familiarity with the condition basically have him pegged within five minutes of meeting him. I personally have been trying to convince them for well over a decade, which means I get to do the "I told you so" dance every time this happens in my presence.
He had a fairly difficult childhood, a fact that I was very aware of. However, as far back as I could remember I only ever heard my parents talk about how wonderful he was. My 4 or 5 year old brain could never fathom why they were constantly singing the praises of someone who was clearly a total screw-up.
They also had completely different standards of behavior for him than they did for my sister and I, meaning he constantly got away with shit that earned us epic punishments. My sister was old enough to figure out that what sounded to me like bragging was attached to the end of statements like "Sure he just got expelled from preschool, but..." While she just resented the unfair treatment, I developed a deep-seated and mostly subconscious resentment of my brother. I didn't realize any of this until we were in high school, and my relationship with my brother had suffered for years because of it.
My parents never got me or my brother diagnosed with anything because they said they didn't want us to have "labels". If you had a "label", people would treat you differently. No shit. If I had been told from the beginning that my brother had something wrong with him, I would have understood why he was treated differently and the constant gushing about him would have made sense. Instead, my kindergarten self could only conclude that they must like him better.