But what did she actually do, this sister in law?
She was apparently rude to her father.
This was called out by her brother, which released a backlash about how the brother has never been there for her and wouldn't even play with her at secondary school.
This is not someone getting their genitals out in public. Being rude to someone is very much in the eye of the beholder, unless she said something like "you are a ridiculous pathetic little man", but as OP says her SIL isn't vindictive or mean this seems unlikely. Perhaps it was more of an offhand rudeness - not thanking someone for a cup of tea, or not saying please, or talking over them, or something that could very easily be done without intent by an autistic person. And rejection sensitivity is very real, as a response based on years of small incremental trauma (and harking back to feelings of abandonment at high school sounds like a response based on deep unresolved hurt).
None of this is OP's problem of course. What is problematic is that rather than move on and see this in any way through the sister's eyes, she is wanting to go back again and find a way to "call out" the sibling for being a twat using autism as an excuse for bad behaviour.
Well the thing is, even if that is what is happening, no, you can't call someone out at an existential level like this and have them say "oh, my mistake, I do apologise" and have peace restored. It will only inflame things further.
They don't have to see the SIL.
It is also one of my biggest fears that my autistic son, who has substantially less insight than you do, will die alone in a rank and disgusting flat and not be found for years. If his siblings can't look out for him, this may well happen. Is it bad behaviour when my son doesn't buy birthday presents for his siblings but they do for him? Because stuff doesn't matter to him much, birthdays are just the day you happened to be born and nothing special, and he is too anxious to shop online as he can't deal with the promotional emails that hit your inbox after? Is it bad behaviour when his brother gets home from uni and he doesn't come and greet him straightaway because it takes him 24 hours to get used to the change? Or when he doesn't say goodbye, for the same reason?
He often behaves in a way that looks rude or uncaring, but makes sense from his point of view. Of course he can also do selfish things, like drink 6 cans of coke bought for the family on his own. And he gets told. But he isn't mean or vindictive.