My brother has severe autism, Ive spent my entire life never hearing an lingual sound from his mouth, looking at his blank stare in my face wondering if he even knows who I am.
When we go to restaurants with the family we ask to sit in the most remote corner and aim for places without a crowd, we go to empty parks to spend time together and sometimes just sit in the car for hours with him, he prefers that. Hes 30 years old now, and I still am unsure if he knows who I am but I love him, hes the only brother I have. He likes to spend hours in the car with my parents, sometimes he just wants to sit and never leave, maybe because he feels the comfort from his childhood when he was able to live with us....who knows
We like to respect other families on a Sunday. We dont want to give them an uncomfortable feeling on what might be a limited and otherwise pleasurable occasion for them. Perhaps we dont feel its appropriate to compensate the shortcomings of him and ourselves by rubbing it in everyones faces, just because we can, or people think its OK we do.
The OP letter was a fake, or a sick joke obviously, but maybe when you come to live with someone dear to you on the other side of life, you become a little more sensitive on how nice it could be to have a "normal" family, and learn to appreciate it enough to be happier leaving those other ones to themselves.