I truly wouldn't value a relationship with someone who couldn't accept me & my family just as we are.
Regardless, for her to have even considered this as an issue in amongst all of the other relevant things she could be worrying about suggests that maybe it is she who has the problem with breast feeding. The likelihood of those particular family members seeing this one specific person at the exact time they happened to be feeding, when they are supposed to be watching a wedding- not exactly a scenario that most people would have concocted in their head unless they themselves were wary of it. Maybe she pulled the 'it's my day' card to make a point that she otherwise wouldn't get away with.