My MIL especially just can't seem to leave them alone - it's like she ENJOYS bossing them around. Do this, don't do that...I wouldn't mind if we had gone out and left her in charge, but it's when I'm present. Often she interrupts me telling them off, so that SHE can tell them off.
Matters came to a head on our last visit to their home. Our 4-year-old had launched into a tantrum (wanted to snatch a toy from her younger sister and I wouldn't let her have it). I was just calmly telling her that if she kept on trying to snatch it, it would be naughty corner time, when MIL starts talking over the top of me. (Shouting, in fact, not in a nasty way, but her approach is to shout in a jokey way, saying "Hey hey, what's all this noise?"
I couldn't help it. I said: "Can I deal with this please?" She walked out of the room!