It is really up to her to tell him.
If your sister knows about your autism you should tell her .
A lot depends on your relationship with your sister, how she views autism etc. If you have a poor relationship with her, autism is probably not the best topic to start improving that relationship.
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Professionally Identified and joined WP August 26, 2013
DSM 5: Autism Spectrum Disorder, DSM IV: Aspergers Moderate Severity
“My autism is not a superpower. It also isn’t some kind of god-forsaken, endless fountain of suffering inflicted on my family. It’s just part of who I am as a person”. - Sara Luterman