In my writing I have a few characters with aspergers. However, they live in the late 1100s. They are siblings, and their father is relatively wealthy, but not nobility. The three have moderate to severe aspergers, that mostly manifests as a high degree of introvertedness, an incredibly strong sense of right and wrong, a slight obsession with one subject, and a tendency towards bluntness, regardless of status or situation. How might this be explained and perceived at the time? What about for his older sister?
The men would probably be regarded primarily as a bit eccentric. This would be well before the ages of standardized diagnostic criteria and categorization based on these traits, and as Aspies they'd have average or above average language development, meaning they wouldn't be flagged in the same way that a nonverbal autistic would. As reasonably wealthy men but not nobility, I expect they would probably be merchant's sons and would be involved in that business. If their special interest and slight obsessiveness was aligned with business interests I expect they would be seen as shrewd businessmen and their social difficulties and bluntness largely ignored as "just their way." If their special interest did not align, I would expect they would come into conflict with the family's goals and see chastisement for having their heads in the clouds and ignoring the important things for [x] thing instead. Either way they would probably be more in charge of bean counting things and less called upon to deal with any negotiations, as they could get in some serious hot water by being blunt to a noble, etc. Their strong sense of justice could also be an issue if they are cheated by someone, especially a nobleman, and don't let it go. However, their wealth and comparatively maskable characteristics would blunt a lot of the effects of being non-normative, and NT normativity was not nearly as powerful a force in the 1100s, so they'd probably do okay. It would likely be explained as just personality traits vs. as part of some condition or as possession. Humors may be involved at the most "medical" explanation, with possibly some attempts to "balance" the humors.
The older sister would have more difficulties. Things such as bluntness and introversion would be qualities which would make her less marriageable at the time, which would be a problem for a wealthy woman, and she might see a need for a higher dowry as a result. Given Aspie tendencies to question social norms and normative gender roles of the time, I see her as fairly likely to chafe in her expected role, and not to be happy about the injustice of it all. She would be much more likely to see friction with those around her and to struggle with the consequences of being non-normative, and more likely to have her behaviors demonized or medicalized in as much as medicalization exists at the time. However, there was still more flexibility in such things than in, say, the Victorian Era, and I wouldn't expect to see her confined in an attic or anything, but I would expect that she might see odd 'treatments' or exorcisms prescribed to try to normalize her. (Something which, sadly, is entirely too relatable nearly 1000 years later.)
Your children would probably be great candidates for monastic orders
Monastries were not as popular in the 1100s as movies might make you think, but if people showed a strong sense of academic skill, particularly in maths or writing, they would have ended up coming to the attention of a priest at some point, who would have in turn suggested that these children be entrusted to God for development of their 'gifts'.
Universities were a long way off (at least in their current incarnation - Oxford was a seat of some learning from 1096 onwards) and many people who were not of the nobility who were 'educated' were educated at a monastery. Children who were precocious enough to learn to read or do their father's accounts would have been ideal candidates for the learning that a monastery of the time would have provided.
Their introversion wouldn't be an issue there, and bluntness would be 'trained' (read as whipped or beaten) out of them. Their sense of right and wrong would have been focused on God and the interpretation of scripture.
Ironically, such a fate (while depriving them of a normal life as it would have been understood back then) would have resulted in them being revered in the wider community and respected for their abilities to read and manage funds. They would be scribes, accountants (or what passed for them back then) and other pursuits, utterly trusted because of their vows.
In short, a monastic order may well have been a great way to harness their precociousness and focus them on tasks that would have been of material benefit in a larger society.
Don't give into the temptation to impose 21st century understanding and compassion on 10th century peoples. How would it be perceived? Not as a disease, or a disorder, or a condition deserving of understanding and compassion. You're talking about a people who thought (at best) that if something was shaped like a human organ it obviously would help that organ including cure it of conditions.
Wealth, than as now, is a great cover-up. Wealthy people are forgiven their eccentricities. If the children are socially functional (which you seem to describe) then their behavior would be accepted without question with the exception of an eye-roll when they were out of sight.
The sister wouldn't see anything out of the ordinary. She would have grown up with it and would perceive it as normal family life. Missing 1,000 years of community education, she would have nothing (nothing at all) to compare her brother's behavior to that might suggest something odd. Remember: wealthy children in the middle ages didn't play "with the masses."
Our modern society educates and socializes children so much more thoroughly and so much earlier than the children of the 10th century (even the wealthy) that noticing behavioral complications like Asperger's is "simple." It's more accurate to say it the other way around: children in the 10th century (the middle of the dark ages!) were not educated and not socialized anything at all like children today. Only the most extreme cases of Asperger's (or anything else) would have been noticed or cared about.
Frankly, this sounds like you're trying to crowbar a social issue into a story ("I want to raise awareness about Asperger's and use a medieval story as the vehicle!"). In other words, you're putting the cart before the horse. Considering how poorly Asperger's and similar issues are understood by the general population today, if you wrote your story with an accurate reflection of how people of that day and age would view the behaviors (behaviors they didn't understand at all), your average modern reader wouldn't even understand what you were trying to preach. And if you try to make it clear, you'll ruin the context of your story such that people won't want to read it. Food for thought.
In medieval times people with mental illness were usually seen as touched by demons or worse. If they're meant to be the good guys you could have people say they were touched by an angel who was in such a rush he forgot half the blessing. If you wanted to make it a little funny.