Just to add - i am sure my mum in law is autistic and she is nearly 80.
When i first met her, she was in her early 40s and married to FIL. I think she got away with passing for NT as her social life revolved around his work events.
So - when i first met her she came across as someone who was very supportive of her DH. They were always very polite to each other and there was none of that affectionate banter you get between loving couples that have been together years.
But, as i came from a family where my parents were always at each other's throats, i thought it might be normal.
When FIL died, i spent more time talking to MIL and her personality has come out more.
Here are her 'traits'
Hates surprise
Fussy eater and high anxiety over food
Writes down topics of convo beforehand before meeting friends or convo dries up
Eye contact with me is poor (this might mean she doesn't like me!)
Would hate us to turn up unannounced.
Never ever gossips. About anything.
Convo lacks reciprocal quality as she does not ask questions that follow on from the unfolding convo. It is like she follows her list of 'how are you, what have you been up to' and then goes to comments about the weather or her garden.
Her life has been one of very safe routine. She met my FIL aged 15. She will only drive 2 routes (and this has been the case since she was in her 40s!)
Playing any fun games at Christmas is impossible. Even with close family members, she gets too anxious. She never seems to be able to relax and laugh properly. She is always worried about making a faux pas.
She does have friends but sees them one at a time and always very formally arranged. Like if she sees someone every 3rd Tuesday, she would stick to that.