I'll try and be as brief as I can without drip feeding.
My SIL is addicted to gambling, namely the poker machines. I don't know how long it has been a problem, but I suspect for several years in one form or another.
Her and my DB are in their mid fifties. Their DCs are grown up and left home. SIL has always been a SAHM. My brother retired last year and it was only then he discovered that his wife was putting sometimes £1,000 a day into the machines in various arcades around the city. I guess while he was at work, he didn't notice. 
They are very well off. He made a lot of money as a lawyer but it's not a bottomless pit from what he's told me. They are mortgage free and each drive a prestige car. From the outside, it all looks great, but he told me after a few scotches at Christmas she has ploughed her way through an 'obscene' amount of money. From a man who probably made in excess of half a mil every year, that must be a lot...
Anyway, once her problem was discovered, he went mad. In some bizarre logic, took her to vegas, so she could have one last hurrah, and then made her go to GA. She has been going to the meetings etc.
As far as he is concerned, she has it under control. He is back doing a lot of pro bono work at the moment (he got a bit bored I think and there was probably tension at home for obvious reasons), but now SIL is on her own a few times a week.
My DH's brother works PT in a big amusement arcade about an hour away. (My SIL wouldn't know this as she doesn't know him, and besides, he's at Uni and this is just his casual job).
He told my DH that my SIL has been in "every day that he's been working anyway" and she's one of the hardcore brigade who have a person assigned to look after their machine if they go to the toilet. AND THEY BRING HER FREE SANDWICHES FROM SUBWAY!
He knows it's my SIL he says, as she parks her black SLK outside! (Possibly being judgypants here but I don't think many people whacking their money into one arm bandits drive £60,000 cars). He remembers her kinda from our wedding and he didn't know about her problem- so was quite surprised.
Now I know, even though DB has told me details (albeit when he'd had a skinful), this is essentially a private family matter between them. But do I tell DB? Do I stay out of it and hope that she gets caught again as is quite likely? I hate knowing what I know and even though I'm pretty certain it's her, I can't be 100% as have not seen it myself.
DH says I should speak to my DB. But we're not that close that I can just call him up and tell him anything.
I have considered mentioning it to SIL but that sounds like an even worse idea..
She clearly has a problem, but then they'll have an even bigger one when they realise she's gambled the house. :(
Also, DBIL will lose his job if it comes out he's been talking about the customers!
Aaargh, WWYD?
I'm posting in high traffic AIBU in case anyone has experience of someone with a similar addiction and if they had any advice?