NTA. I don’t think it’s ableist to want to enjoy a night you’ve worked so hard for & should be able to celebrate. Someone else mentioned that your sister may be acting this way out of jealousy. It’s possible. Is the party invite only? If so, stand your ground & make it clear that your sister is not invited & that if your parents insist on bringing her, then they aren’t invited either.

“Usually, this leads her to belittling me and being mean. My parents always tell me to get over it, and that since she's autistic, she can't help it.”

I think the reason she “can’t help it” is because your parents most likely never tried to teach her differently. I have 2 kids, one of them is autistic (8M). Sympathy, empathy, recognizing social cues don’t come naturally to him. I have to actively teach him these things. And you know what, he’s learning. Sometimes he can be mean & I have to decipher whether or not it was unintentional due to his lack of filter or whether it was very much on purpose. Just because he is autistic doesn’t mean he can’t be held responsible for his actions. I think your parents need to learn that as well. Autism shouldn’t be used as an excuse to be rude or mean.

Enjoy your art show. Congratulations on your accomplishment!