Identical twin or non-identical? Either way, you're a different person from either your twin or your younger brother and whilst there's probably an inherited component to Asperger's syndrome/Autism the fact that they have a diagnosis should have no bearing on your medical if you don't have it.
If you can rub along with people OK and remain reasonably flexible in your thinking, then I don't see that you should have a problem. If those with family or personal histories of Asperger's syndrome were denied medicals, the skies would be a lot emptier.
(Not an AME, I admit)