How do you let you mum-in-law know that you will be calling the shots on the upbringing of your child (without sounding rude)?

I think ground rules need to be set very early on, even during the pregnancy itself so that your mother in law doesn't feel blindsided by the lack of control she has over the situation. It usually sounds rude when both parties initially don't discuss their unhappiness with each other and keep it bottled up. This does not allow for each party to clearly state why they think the child should be raised a certain way, and therefore no one can understand each other. While you should be the person calling the shots, I wouldn't reccommend disregarding their advice completely. They raised you and you turned out alright (hopefully). So there must be some wisdom there. Of course there will always be disagreements but what's more important is that these disagreements DO NOT happen in front of the child.