How can I resolve this problem with..?

Okay im a working mother of three kids. My mom died when I was only 9 and my dad got married 4 years later. Okay the problem is my stepmother who comes by once in a while to visit or babysit for me. She always changes everything I do around or criticizes me if I discipline or punish one of my kids.Everytime I say to her dont give them any junk food or dont let them stay up too late she lets them do it anyway. One time she even cleaned out my whole fridge and threw out everything she claimed was "unhealthy" In my perspective she is very hard to please. Every since she was in my life as a kid she has always put me down. She even says that the way I parent my children is bad or not the right way. Shes very difficult to please and always says the things or choices I do for my kids are wrong or not the proper way. Shes very snobbish to me and likes to have an opinion to everything but she spoils my kids and my kids even call her grandma (since they never met their biological grandmother) she does treat my kids nice and is very helpful to them with hw,cooking,etc. How do I tell her or let it get into her headthat I am the mother and that what I say goes? I talked to her a million times and she doesnt seem to get it. I want her to realize that what I say goes and not to go behind my back or sneak around and go againest my rules and change what I do or did for my kids.