From SIL POV: You’re the daycare. Except you don’t cost anything and you’re family. Basically a big win-win for her. She’d be crazy to not take advantage of this situation, and I’m guessing she even means well.

Your husband’s POV: He gets to spend time with the kids of his sister, he’s entertained and he feels good about it. He’s helping his sister and isn’t bothered by it; he actually enjoys it. His mistake is not consulting you.

Your POV: You don’t want to unexpectedly share your home with someone else’s children. And you’re a bit insulted that your husband isn’t at least letting you know about it. I don’t think you make any mistakes in this matter.

I’d try to solve the problem first by simply communicating the problems with him. You know best what arguments easier hit home with him. Some men respond to emotions, some respond to good arguments, some require alternate solutions to the problem.

• Emotions: Just get mad at him and let him know how strong you feel about it;
• Arguments: “You wouldn’t like it if I surprised you with loud music when you don’t expect it” and “It’s our house, I also take care of the house, the income, everything. I have a say in the matter”;
• Alternate solution: “Only on Mondays and Saturdays, any other days first go through me. And your sister takes all of us, kids included, out for dinner once a month on her costs; let’s keep the family social and together, just just a trade of kids.”

Alternatively, if your guy won’t listen you should go complain to the SIL instead. Tell them both that you feel pressured and uncomfortable with the unexpectedness of it all; you have your own life, and you don’t want a surprise visit of kids in it whenever they please.

If all else fails, every time the kids are there you go to a hotel on his costs to get your peace and quiet. Hell, dinner’s on him and so is the cinema. Add consequences to his actions.