If you don't look at the eyes of other neurotypicals, how can you tell that your siblings' eye movement is different?
It's known that autism is genetic, so it's very likely for siblings of someone with ASD to display autistic traits (Broad Autism Phenotype) that would explain the unusual eye movement.
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"They sound good in my brain, then my tongue makes not the words sound very good, formally." - Homer Simpson
Undisgnosed - Aspie score: 122 of 200 - NT score: 105 of 200