ooooh gosh... that would BE SO ANNOYING!!! You know, I think you really should say something, and not be "so nice" about it. Yeah, it may hurt your mother in law's feelings a bit, but she will get over it. You should let her know how you feel and how much extra unneeded stress this is causing you. You should also let her know that you respect her opinions but you are the one who is going to the make the final decisions on YOUR daughter! (btw your situation sounds a bit like the movie "monster-in-law"..hehe have you seen it?
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Good Luck!!