AITA for not wanting my autistic cousin at my child-free wedding?

Throwaway because I have family that use Reddit as well. Next spring, I am getting married to the love of my life. We have decided that we don’t want children at our wedding. Not many of our friends and family have young children, and the ones that do are fine with getting a sitter for the weekend since we gave them a lot of notice.

The only ones to put up a fight are my aunt and uncle who have a daughter with autism. She is 20, but will be 21 by the time my wedding day comes around. She is what they call “high-functioning”, which means she can talk and wash/dress herself, and she has some friends. She graduated from high school a couple years ago and is currently living with her parents (my aunt and uncle) while working at a grocery store. Despite her being technically an adult I just don’t see her as such. Every time I talk to her at a family gathering it’s like talking to a child.

She is obsessed with toy ponies and barbie dolls, and brings them up at every opportunity. She draws in her notebook constantly and never makes eye contact when talking to me. She does not have loud meltdowns like other special needs kids I’ve met before, but I really don’t want to risk her ruining my special day. I told my aunt and uncle that I didn’t want her there and they became very upset. They said she already saw the invite and knew it was child-free but because she is an “adult”, she thought she was still included. My aunt tried to guilt me by saying she’d already picked out a dress and a gift but I didn’t want to hear it. My fiancé says I’m being an asshole and we should let her attend because she has attended other weddings before with no issue. Everyone is making me feel horrible for not treating her like an adult when she doesn’t act like one. AITA?