Motherinlaw blamed me for my childs overweight problem?

Much is genetic. Although there are 70 genes that can help lead to obesity, the impact of diet and habits are also key. Tv, ads, and fast food brainwashes kids to crave burger, chips, and candy. It takes great effort to train kids to really crave apples, green beans, and carrots. Much harder with this generation. But at least worth the effort.
Take the family..... Take your child, your husband, your mother-in-law, and any other person involved in the care of this overweight child to the doctor. Get professional help from your pediatrician or get a referral to preventive cardiology (or pediatric obesity). With everyone in the same room at the same time, ask how to help the child. It isn't about blame, it is now "all about the health of the child" now!