How much would the label really change your situation? Forget the labels and focus on the symptoms. Being told that you have "tendencies" does not equate a diagnosis. I was told in the past that I had "schizoid tendencies", but that did not equate to a full diagnosis. As long as they do not affect your life in a significant way, just accept that you may have some autistic tendencies and then carry on.

It is definitely not equal to being called "retarded" and I would ask you if you consider all people with this diagnosis to be retarded, or if you are just being especially hard on yourself? If you would not call someone else with autism a retard, please don't do it to yourself.

Sibling's mental illnesses cannot be passed onto you like they can be from a parent. But if one sibling has it, it would not surprise me if you are more likely to have it, or even just tendencies.