I also think it is a boundary violation. If she wasn't in your home and in a neutral place, then yeah, it gets blurry but she's in your home and so that changes everything, imo.
However, I think you could probably defuse the whole thing with a little humor. Say what you gotta say but in a humorous way. This allows her to save face and doesn't put your wife in an uncomfortable position but it also gets your message across loud and clear. Something like "Do you have potatoes growing in your ears?! I just said they are fine.." and move on.
The downside of handling it this way is that it may take longer for her to realize that you are setting a boundary with her, than it would if you used brutally direct language. However, it avoids the "what's up chrome's ass?" convo that she would no doubt have with your wife, later on.
I doubt she means to meddle, it's just that a pattern was set early in in your R (by yourself and your wife) and it happens to be one MIL likes because it affords her a position of prominence in your lives.
Well, that's my two cents..use humor to get your point across without causing a major blowup. But you know..to correctly pull this off you will have to have a smidge of aggressiveness in your voice; it can't be all laughs and giggles. I'm sure you kwim.
Good luck. Btw, I have MIL issues too and it is verrrrry tricky to navigate these waters. I'm probably not qualified to dole out advice but, then again, who is on these boards, LOL.
