I understand it. I have emigrated like your sister, only that my relationship with money is not the standard. Given the terrible inflation, I was used to live without any savings... I was making 500 USD per month, and spending everything. Some (poorer) friends perhaps only liked me because of that. I think it's not like I was buying their love or anything.. just didn't mind. Tried to be quite stoic/ anarchist about it. But then, that's me.
The standard (capitalist) way to go about money is that it's yours as your soul. It's an essence. You don't share. Unless when you do a gift (e.g. birthdays)... And, even couples split tabs. Also, the standard (patriarchal) way about money is that Males pay. So your brother in law is just spitting the misery cassette.
We (commentators) can blame all we want on this people. It won't make OP feel any better.
OP: I think that a difficult talk with your sister and/or brother in law is due. Perhaps even a letter (you can also ask someone else to proofread/ help). Tell them how you feel... That way, they'll know why you are not visiting so often. If they are really shitty about it and get mad or anything else other than comprehend, then that's onto them.
PS. Do they have kids? I am an uncle, and my niece is in another country. But thankfully, since I emigrated, I have a not-so-dissimilar salary to my brother so I can plan holidays together... I'm actually looking forward to that. It will be our first.