I can't offer any suggestions, but I can tell you that both kids are at the age where rivalry comes into play. I'm the oldest of three kids, two NT and me, an Aspie. My younger sibs fight all the time. The youngest just turned eight and, until the time she was about six, she never yelled at my lil bro or fought with him. Now it's constant. I think it's just a thing all siblings go through. I learned to cope with my brother and my rivalry and fighting by ignoring him. It took a long time, but it did work. I don't think my parents could have done anything differently to make us fight less, and we still fight sometimes.
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Without the weird people, how could anyone define normal?