Ditch the belt and cardigan altogether and add a fitted black blazer that visually cinches the waist even when it's left open and then some black heels.
Oh and a different jacket that isn't sweater material. Maybe a pretty scarf to put over your shoulders, iron the dress if you bought it and trade that belt for a skinny belt of some sort.
Well it looks like you're in a dressing room so I'll discount the hair and bra straps, but it really depends on the wedding and your age. If it will be a easy going, casual spring wedding then maybe, but even then I'd stay away from denim/chambray.
It really depends on the wedding. A lot of people go casual if the wedding is casual. Not everyone has the big princess fairytale wedding. If your cousin is okay with casual attire then the dress is ok but lose the belt & find a better jacket/shrug/cardig
There's a lot going on here. I'm guessing you need to keep the belt, so lose the necklaces, and headband, and let your hair down a bit. Not sure about the colors, but it looks like tan & gray, which is pretty dull for a wedding. Try a colored jacket?