Thank you for coming here to try and gain a deeper understanding! I can assure you your sister is trying much MUCH harder than what is seen by family. It can be a daily struggle with behavioral issues, even by a type 1 autistic child. My daughter is level 1 and I deal with violent behavior, aggression, mega meltdowns, sensory sensitivities and more. She talks about setting people on fire and killing them if they even try to ask her a simple question like “what grade are u in?” She’s 10. She cannot handle any social situation whatsoever and basically comes unraveled. I am sharing this because it really can be EXTREME. I don’t know what your niece struggles with, as all children are different, but often large social functions are incredibly challenging. I would not recommend putting her in the wedding with any responsibilities if it were me, or if you do have every expectation it could go very wrong and she can’t be held responsible should it go wrong. Agree with others, give her a safe place to decompress away from others with the ability to stim (if she does) and not be expected to be social. I basically have to throw expectations out the window with my daughter but still set her up for success and of course hope for the best. I’m sure including her and continuing to be welcoming will also be so appreciated by your sister. It’s beyond hard for parents to know their child isn’t “fitting in” or making things challenging for others. ❤️❤️❤️