Why do mother in laws always think they know more than you?!?

She is VERY overbearing and we haven't even told her we are expecting yet. I have a 5 yr old from a previous relationship but my fiance is the only father figure he has. We are very exciting to complete our family with this baby but I just know she is going to aggravate me. She is the type to but in bc she thinks her way of doing things is the better way. In alot of cases she is wrong and usually I do not say anything bc I do not want to offend her. My fiance will of course say something, but thats her son so of course it is different. She is the baby hogger type and I am stressing already! She is the type where if the baby cries she will make a point to run there first then act like we're doing something wrong. With my son, if he is in a "time out" and is crying she will run over there like we're horrible parents or something. Ahhhhhh what am I going to do!