Yes, you have to talk to her or it'll fester and get worse. It doesn't need to be an argument, but if that's where she takes it, so be it. But like affairs where so many people want to make it about the sex (which is totally irrelevant in the grand scheme of things), keep this about what might be the bigger issue - her inability to talk to you. Nobody's perfect, so you're mother may not be a 5-StarChef, but I'll bet she is... Being critical of others isn't ever a good idea, but letting it extend to the family of the person you're going to spend the rest of your life with, and whose family is prolly part of the package, is scary.
Sorry to say it, but what you're seeing here might just be the oil and water that you two are. IOW, it might not be the family at all really..
Oh, and if invading their privacy to find out the truth is the only way, so be it. Just be ready for that to be the focal point of the conversation, not her dislike of your family, it's an easy out for her. Deflection of the truth, it's a common tactic...
Maybe she knew you would snoop, and decided this is just a simpler way to tell you.. ???