That really does create an interesting situation, to have one NT in a group of 4 children. Am I correct that one or both of the parents is also Aspie?
I have one of each, NT and Apsie, also boy and girl, and I think they learn a lot from each other. There is a very strong - and normal, lol - love / hate relationship between the two. The good days, when there is more love and very little hate, are really fun to see.
I guess my main question about your NT child is, does he resent being dropped into the wrong family, or is it more of a joke from him, a way of pointing out the obvious to remove the weight of it? It would rather depend on the tone and context of the statement, I think.
As for your daughter, with such an experienced mom, I am sure she will be fine. Great, even.
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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).