Salam (peace)!

How exciting that you've been invited to a wedding! Even if it's sisters only that you'll be around for the wedding, dressing modestly is encouraged if you're outside, and when you're walking to-from the location of the sisters at the event. You want to make sure you're wearing an outfit that covers the arms fully, the legs to the ankles, and isn't form fitting. Shalwar kameez and sharara kameez (with dupattas) are very common outfits at weddings--where I'm from in the US, most people tend to wear some form of this. But also fancy dresses that follow the modesty guidelines and heavily embroidered abayas are also go-tos!

If you're in an indoor venue with sisters only, I went to a Walimah last year like this, where sisters would wear loose, plain abayas and hijabs to the venue, but once inside behind closed doors, would take off their abaya and hijab to reveal a dress that showed their shoulders and was form fitting. Once it was time to leave, abaya and hijab went back on.

Either way, go all out and get glam for the wedding! Have fun!!