MIL invited herself over

My twins are having their first birthday in November, and seeing as my SO's family lives a good 16 hour drive away, I told his mother in advance when the party was. I got a call from her a week later saying she was just going to stay with us that week (yes, she wants to stay Nov. 4-12) because she can't bring their travel trailer and doesn't want to pay for a hotel that long. She didn't give me any room to say no really, and even when her son tried to tell her it isn't a good idea, she shut him out about it. The biggest reason we don't want her here is that she just isn't good with babies and we don't think she'll be considerate of their schedule. She's bringing her 13 year old son (technically her grandson who she adopted) who has severely under-managed autism and that makes us so nervous because he gets violent and rambunctious so easy, which scares us considering we'll have two babies crawling/walking around. It wouldn't be such a concern except she doesn't ever try to help the situation when her son gets in a mood like that, all she does is yell at him to stop, or smack at his arm, sometimes she just ignores it. I just don't know what to do. Our family survives because of the schedule and routine lifestyle we have and I know with them in our home for a week it would all go to hell in a hand basket. Please someone give me some advice on how to handle this.