Well dude, don't beat yourself up. I figure being an older sibling is a lot like being a parent (in kind of mini-version anyway). Nobody gets it perfect, because you have to do it all the time and you're learning as you go. On top of that, older siblings are just kids, learning how to live and be a human and all that, whereas at least parents are (or are supposed to be) grown-ups.
So cut yourself a break on the big brother front. Because most likely you do a pretty good job.
Relearning about ASD-- Look, everyone is relearning. Seems like even the experts need to relearn stuff; we are all relearning as we go along. Just don't get so attached to what you have been taught that you cannot see the people right in front of you.
From some of the hell I have been through (and put my kids through) I would ask you as a behaviorist to be careful not to fall into what I call the "black box trap." Emotions aren't your business and you are not supposed to concern yourself with them; you are supposed to concern yourself with training behaviors, with reward and punishment, with stimulus in and behavior out.
OK, I get that. But please remember, as you go along, that those "black boxes" are people, that they do have emotions, and that even if those feelings may not be the stuff of your business, THEY ARE IMPORTANT.
If I hadn't spent years caught (sometimes by others' force, and sometimes by my own) in the "black box trap," I would actually probably be happier, healthier, and more functional today. The Skinner treatment may "fix" the behaviors that are annoying or disruptive or disturbing to others, but it actually DOES NOT do much for mental health, or empathy/sympathy/compassion, or the ability to give/enjoy emotional relatedness with others.
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"Alas, our dried voices when we whisper together are quiet and meaningless, as wind in dry grass, or rats' feet over broken glass in our dry cellar." --TS Eliot, "The Hollow Men"