I sit alone sometimes too. I'm not always in the mood to pay any mind to social codes and language and all that non-verbal behavior stuff and what other people might think of me, so I DON'T always flip my social switch ON and I sit alone with my thoughts.
I don't like it when people sit next to me and then all they want to do is talk, talk, and talk. Then I feel like I have to be social and talk back when I don't always feel like talking and listening. I will sit by myself and away from others when I feel like that, in hopes NO one will come and sit next to me. Most of the time it works but I serious believe some people act like me sitting alone is a beacon signalling them to come sit next to me and talk.
I believe your sister questioning your decision to sit where you did is ridicules. You should be allowed to be yourself. You weren't hurting anyone. Even if you hadn't paid any particular attention to where you sat, I believe it's silly to try to make something of it.
There is a wonderful old man who's name is Charlie and he lives across the yard from me. He likes to sit out on the porch and smoke his cigars or pipe and watch the World in peace and so do I. We always acknowledge each other with a silent wave. Maybe he is an Aspergian too, I don't know? I had one neighbor comment about how strange me and Charlie are, always waving but not speaking to each other. It really irritated me. Why do some people think there is something wrong with you just because you don't always want to sit right next to someone all the time and talk for hours!?!
I'm so glad that woman moved and took all her problems with her.
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Whatever you think you can do, or dream you can, begin it. Boldness has genius, magic and power in it. ~Goethe
Your Aspie score: 167 of 200
Your neurotypical (non-autistic) score: 35 of 200
You are very likely an Aspie.