Dh's sister has announced a visit this weekend, AND that she is bringing a friend. (A friend I sort of know (met once and liked) but...still.) DH told them that our "rule" is that one person can stay here, but if two or more people visit, they need to stay at a hotel because our place is too small, DD is a light sleeper, etc. Of course, DH's sister said they "can't" stay at a hotel, and really pushed to stay here. It's going to be *so fun*. P.S. his sister is *extremely* mainstream and rolls her eyes at almost everything we do. And she calls my MIL to "report" news that happens here, and give her the daily scoop.
Why doesn't your DH put his foot down? If that's the rule, then that's the rule -- no discussion. You teach people how to treat you, and what you guys are teaching your SIL is that she can just steamroll you and you'll cave and give her what she wants. And when she leaves her hotel to visit your house and starts rolling her eyes at your parenting, say, "If you're unable to respect us in our home you'll need to leave." And mean it.