It's all in what you consider important. A lot of folks wouldn't agree with me, but I figured when I was married, that there were other things a lot worse he could be doing. His bad habit was gambling. Which was a vice I was already aware of. I "confiscated" money as soon as he was paid to take care of bills and such. This was an arrangement we reached even before we said "I do". I kept a "household expense log" and showed it to him each week, so he was fully aware of how much things really cost and where money really needed to go.
If your sis considers him worth it, maybe she needs to sit him down and calmly discuss finances and make different arrangements than what they currently have. She gets the money first and then gives him his gambling allowance. :shrug:
I have to admit, being married to a habitual gambler can be rough. Yet, if you are fully willing to reap the rewards when he is winning big, then you should be just as willing to suffer the consequences when he is losing big. :shrug: The ex used to say 1) you can't win big unless you bet big and 2) you can't win if you are scared of losing your money.