They sound more like they are in denial than blind. They can see the evidence but don't want to believe it sort of thing.
This is likely to be a delicate situation. Tread carefully, do your best to avoid raising emotions.
One thing to consider is that, would knowing that your mother has AS really help her? If not, perhaps it is best to drop it, since you'd be arguing about something that has no benefit...
If you think it will help her though, then help her.
Best of luck.