How much would it cost for a cheap-but-cheerful motel?
How on earth are they having a lie-in every day with a baby?
I'm not surprised to hear that the drinker is blaming the one who is incredibly vulnerable, i.e. your sister alone and without support. I think you can only tell her that you're concerned about parallels with your own marriage and ask her to think about whether her husband's behaviour is reasonable.
Beyond that, and minimal civility, I don't know why you give them any headspace at all. They expect you to attend a cooked breakfast: don't. They expect you back for dinner: don't go back. They threaten your children with the police - well that would frighten any child, not just ones with actual dealings with the police. And is wholly wrong. Just call them out on it. Don't be polite - they aren't.
It sounds like BIL isn't speaking to you anyway - useful. If sister criticises your children or interrupts you, just clearly state your position "please do not interrupt me/please do not make critical remarks about my children". Calmly and assertively. Broken record technique.
I can understand why you don't want to fly home early, but I would seriously throw a few quid at another place to stay for the last week.