There is absolutely nothing wrong with what you are doing. I think the only thing I would change is to tell your sister to not share LK info with your mother. It will probably be hard for her, particularly if your mother is grandmotherly toward the other grandkids. She probably expects that your mother generally behaves the same to your kids. I find your mother's immediate comparisions to your sister as very illuminating. If something is like your sister, it is great. If something is like you, it is horrible, and eyerolling results.
p.s. The enemas mentioned by the PP in the 40's were because doctors told mothers to Schedule everything the baby did, and that if they didn't poop at the right time of day, then to make it happen.