It all depends...
Sometimes i dont even know what it depends on. Sometimes i'm okay-ish, and sometimes i can't handle it at all. sleep and food seem to affect this.
I have problems with interacting with those that i want to when in almost overwhelming social areas. And i notice this, usually while its happening, but there's nothing i know that i can do to even express that when in those types of situations.

Could they go there a week or two before the wedding, and you spend time there while it's less overwhelming and different, so you can check it all out, and get used to the area? and maybe walk through the wedding details so you know what's to be expected? and maybe have a place where if it starts getting too overwhelming that you can calm down in? (like one not frequented by others). and maybe explain to your sister that if you suddenly go there that it's okay because you just need to get to somewhere not becoming overwhelming, and that you will return when you can??