Actually, I would say my 8 year old NT daughter would have focused on the sheets as well
Kids are, by nature, self-centered. Just FYI. But it's a cute story
I guess my question is this: is your nephew experiencing any issues that make you think he could benefit from having a diagnosis? Is he doing well in school, is he happy, does he have a positive relationship with his family, etc.? Knowing, by itself, isn't everything; it's what the knowing GETS you that matters, IMHO. Services in school, accommodations, a key to understanding a child that may have had a parent otherwise mystified.
If your child has a diagnosis, odds are that your brother has given some thought, even if very lightly, to whether or not his own child could be on the spectrum. Whether or not he would share those thoughts or do anything with them would depend, most likely, on a variety of factors.
If you do decide to bring it up, start from the assumption that he is likely to have considered it already, and ask questions first.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).