Update for the still curious voyeurs.
I talked to my brother in law (gambling SIL's husband) this week during our annual family beach trip. He says things have continued to get worse but he is persevering.
They sold their unencumbered SUV roughly a year ago for $20-25k or so. Apparently all that money is gone, mostly to pay off gambling debts, but I'm sure also just spent on stuff. Gambling debt is as high as ever - $50k at least from what BIL mentioned to me, but I didn't ask for an exhaustive list and he wasn't intending to provide one to me.
BIL and SIL essentially have separate finances at this point. BIL will occasionally give SIL $50 or $100 to buy groceries for their kids, and that generally gets spent by SIL on food for the kids.
BIL says that SIL is still deluded about how to get out of this mess, thinking that more gambling is the way to go, because that is the only way to get out of this hole. She is hoping to win the lottery (lol). I quickly pointed out to BIL that she would be broke within a year or two even if she did win the lottery.
BIL's focus and goals are to do the best thing for his kids who are in 6th, 9th, and 11th grades. At this point he is of the opinion that keeping the family intact is the best course for his kids. I think part of the rationale stems from knowing what would possibly happen in a divorce court to his finances - outcome could be worse than having the leaking sieve of a wife blowing through all her money. His money remains his, he just has to pay most of the family's expenses. Another motivation for not divorcing his wife is religious in nature I think.
BIL is instituting tough love on his wife. She now has 2 jobs so works 80 hours a week between the two, including a looong 30-45 minute commute to her 2nd job (night shift). Her 2nd job is in a factory, so also requires physical exertion. I think she has been working like this for a few months now. It provides her enough money to pay her gambling loan interest (5% per month on $xx,xxx in debt). And it exhausts her so she is too tired to gamble, and requires her to be at work on weekends and at night so she doesn't have time off to gamble on weekends (when her gambling buds gamble). The downside is that her and BIL and their kids could only stay at the beach with us for approximately 16 hours before they had to leave so SIL could go back to work on the night shift.
Externally, nothing appears to have changed to us outside observers, but internally I think BIL has figured out how to cope with an untenable situation.
Hope you enjoyed your daily dose of schadenfreude.