If you hide away to feed this time you will be expected to hide away at the next family gathering and the next one and the next one and on it goes...
I would start the way you want to. PM her to tell her that you bought a new dress especially so that you could feed at the wedding and that you will decide how to feed based on your babies' needs as the day unfolds (if she has lots of annoying family needing to feed is a great excuse to duck off and hide for a while )