I had to learn this with my mother in law. She used to give me "advice" which were more like demands. I was kind blunt about what I thought one time about what she said, I said it in a nice time and with a smile, but what I said wasn't the nicest. My baby is two now and she still shares her opinion, but doesn't demand anymore.... I just let her know were I stand and that it is MY child. Be firm mama.
In the same boat as you! I just nod my head and do what I want to do in the end anyway lol. It gets annoying and I get worked up. I have my fiance talk to them so that I don't have to.