My DM and DB and Dsis are autistic. I am not. They vary where they all are on the spectrum. My older DB is very limited, my DM functions but struggles in social situations and has no insight as to how her behavior impacts. My Dsis is like my mother but slightly better.
To be honest, and I have only realised recently how badly this has affected me and my relationships with them. Growing up I could not comprehend why we were so different, and why I became the parent. My father left when I was small.
None of them realise how hurt I feel sometimes. They just cannot comprehend it. They panic if I get upset and either explode or ignore the problem and carry on as normal.
As a sibling, the only sibling without it, I struggle. I always have. I became a people pleaser and unchallenging and don't realise how stressed or tired I am until its too late and I find myself strung out and crying.
One of my DCs has it, one doesn't. The one that doesn't is younger and very sensitive, affectionate and cuddly and is always hurt that my older DC is cold, or appears uncaring and inconsiderate.
Out of my niece's and nephews, more have been diagnosed than have not. Again, they vary with how affected/impacted they are from very severe, to more mildly.
We all just fumble along, but I can say as a daughter, sister, aunt and mother of those with autism, it is really lonely and frustrating.