Such a small thing for the "whole" family to be outraged about, IMO.
I'm rather shy, and my husband's extended family is quite loud and boisterous, and when we were first going out, I was intimidated by them. However, I did at least feel like I was being given a chance and that everyone wanted to like me. Now things are fine.
However, if I had felt unwelcome, things like this might have gone on, too. It kind of sounds like the channels of communication have never been opened between these two, especially since the biggest gripe the MIL has is that her probably-future DIL doesn't seem to know how this family runs things, or else is too unsure of how to participate without feeling like she's intruding. Sounds to me like Amy was right on this one, and a little graciousness might go a long way.