My Mother-in-law wants a granddaughter, but she refuses to take care of my sons.?

First, let me ask you something. Why are you trying to have a girl for her. It is apparent that she has always wanted a girl and wasn't lucky enough to have one. So she is trying to get what she wants through you. As a grandparent she should love and take care of her grandchildren equally. They are still her grandchildren regardless of their gender. She needs to get her act together. If you want to try and have a girl, do it because you want to not because she wants one. You need to get it straight from now that all your children will be treated the same. If not you will have troubles down the road.