Line in the Sand

Where do you draw the line? My two kids and I are staying with my parents for a good part of the summer (or shorter I hope). My parents have now taken to overstepping their grandparent role and are assuming the parental role with my kids. They have decided to study for meetings with my son. Last week we hadn't prepared for a Tuesday night meeting. After we came back my son was asked about it. He looked confused when she asked if he had been prepared. She said she would take over the studying with him. I interjected and said I didn't have the material, remember we are just visiting. She basically ignored me and said we'll address it next week. Today she came in from playing outside with my daughter. She didn't like the way my two year old interacted with her and said she would no longer play with her. I think it took me longer to get her dressed than the whole time she played. Instead, while I am putting my daughter down for a nap they snag the son and decided to prepared for the meeting with him. That is so not your place. But how do you intervene when you are in their house? How and where do you draw the line in the sand? I was not given the material to go over the info for tonight. That's how they are using it to get around me. Driving me crazy... How do I put a stop to this?