Good for you - it sounds like you're making a good start. It may take some time to really get the point across, but it sounds like your family will take no for an answer when you're firm.
As far as your sister goes - the reason she can let her husband off from parenting responsibilities is because she can farm off his parenting on the rest of her family. Instead of one parent watching and the other packing, she can send the kids off to your place, and your mother (or by proxy, you) will handle it.
It sounds like your BIL is a father of the old school - what would have been unremarkable 50 or 60 years ago. He earns a salary, loves his kids, will play with them on occasion, but regards anything to do with raising them as the mother's responsibility, and is kind of uneasy being alone with them. If that's the family arrangement they want, that's totally their decision. But they have to handle the results of the decision, which is that the mother does basically everything.
I don't think there's anything wrong with wanting another pair of hands when you're out with the kids doing something. It can be a chance to do something you normally wouldn't do, and a chance to spend time with someone outside of our own house. But - it needs to be voluntary for the person who is coming along, and you can't take advantage of it or demand it.
The OP is running into a classic problem that crop up with doing nice things for people. Often, when you are always there doing the nice thing, it becomes expected, and not appreciated for what it is, and taken advantage of. If you find this happening, it doesn't mean that you have to stop doing the nice thing, but it can help to not always be there on demand so they appreciate it more, and realize that you're doing them a favour.
As far as escaping goes - coffee shops can be a good place to study, if you don't abuse the system (ie, you have to buy something reasonable, and if it's busy you shouldn't hog a table for hours and and hours). In the summer, take your books/laptop and a blanket and go to the park. During a week day, a food court in a mall can be pretty quiet - get a snack and study for a while.