Judging by her message, I believe she's using 'older relatives' as a cover for her not wanting you to BF a toddler at her wedding. I'd go as far as to say I'd bet this message would never have been written if your son was a newborn. Which, really, says a lot about her as a person if that's the case.
I reckon this might be the case. Either that or she is communicating a perceived offense. What sort of older relative would ring a bride and say "hey you know that SIL of yours, yeah if she breastfeeds it's going to offend me. Ask her to cover up please." it's more likely that she just thinks that her Great-Gran doesn't want to see it.

I think a discussion is required. Face to face.

Little bit off topic, but someone mentioned how she's probably uncomfortable with the whole discussion, which is why she's gone the Facebook message route. I really don't like how technology seems to be enabling a level of candor that is bordering on rude. If you aren't prepared to say face to face, looking the other person in the eye, what you are about to put into a text message, email or Facebook message, then maybe you shouldn't be sending that message.