R.W.
Month long visits? That's awfully long. Is it possible to have shorter visits, but more of them? Even 2 visits of 2 weeks would be better.
You have a tough situation. Normally I would say don't worry so much, but I assume you have reasons for worrying (like genetic weight problems or eating disorders or emotional eating in the family).
I was going to suggest ideas (like making a schedule so they cook some nights and you cook other nights) but they may be too domineering for that.
Try to remember that kids have short memories, and although the visits feel long, your own parenting will be the dominant influence on their habits.
When we are on a vacation, or there is some other special circumstance, we just figure, 'well, this a special time, so we can allow special rules' and let it go.
But like you say, it is hard to ignore for a month. If you both agree to be very firm with the mothers, you can tell them that they had their time of parenting, and now it is your turn...also that they are guests in your home and causing you both stress by overruling your household policy, and if they will not agree to some simple rules then you may need to keep their visits to a shorter period to avoid confusing the children (and kids DO get confused about who is in charge!
Maybe the worry of losing time with their grandkids will help keep them in line.
Also, if you can afford it, you could visit them instead, which would give you control over the length of the visit.