If you find any of your relatives or friends getting involved in your personal life you should stay aware and take appropriate actions against the situation.  
If you’re sure that your mother-in-law is doing such activities behind your back then first discuss it with your spouse and explain the scenario. After that, talk to your mother-in-law ask her the reason behind such behaviour and find out her intentions. If still, things do not get on track avoid sending your children to her place and give your child a better environment with positive and good thoughts.