I think my mother-in-law thinks I'm autistic and I let her because it's just easier.
Why do you think she thinks you are autistic?
She has asked me if I am autistic multiple times.
It's really not a good idea to let people think you have any type of disability, if you don't. It's sort of hurtful to those around you who actually do. You should ask your mother-in-law why she thinks that and make it clear that you don't appreciate the questioning.
She took my glasses off once and said "you can see without them, you probably have Autism like your son". It's not something I'm proud of but I can't make her stop.
It might be difficult to get her to stop but you have to do more. People might think that that is coming from you and would be angry that you are pretending to be autistic.