It seems that some NT siblings can "catch" the sibling's autism. They start off as NT then when they hit adolescence they become quite emotionally troubled and show some signs of PPD-NOS-type traits of autism. That's my experience anyway.
My brother started off NT; all through childhood he had normal social development and had lots of friends and didn't have any autism symptoms, until about age 16 or 17, then he started getting depressed, had frequent bouts of withdrawal, and was odd in some other ways. Now he's in his 30s and has got himself a diagnosis of Asperger's, although I still don't think his problems is autism, I think it's more mental health problems like a depressive disorder.
But I think that he is NT but had "caught" some Aspie-like traits because of me, but not actually be on the spectrum.
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Female
Aged 32
On antidepressants
Diagnosed with ADHD, anxiety and mild ASD
I don't like autism being mentioned to me as I'm partly in denial.