Apologies in advance. To avoid drip-feeding this is going to be pretty long.
A few years ago, we found out that SIL had run herself into an epic amount of debt. The kind of amount that if you won it, you might even think about retiring. The details are pretty identifying so I can't go into them but it's enough to say that it was a combination of massive loans and burning through money that had been entrusted to her and she has literally nothing to show for any of it, ffs. Anyway, after she lost her house her DPs stepped forth and put another roof over her head. (I wouldn't have been so kind, but easier said when it's not your child.) All the while, they kept asking, "Have you told us about EVERYTHING you owe now?" and she kept swearing that she had, and then time and again, something new would come out from missed bills to unpaid bank loans. She is, by any standard, extremely secretive and would not have confessed to any of the debt had it not come out by accident, and she even tried to hide what she'd done via some ridiculously crap fraud that could have actually landed her in prison if the relative had wanted to go that far. In short, her finances are shot. If she does nothing but repay her debt for the rest of her life, she might finally clear it when she's seventy. She will never have a mortgage, she'll probably never get credit, and as far as her DPs are concerned, she can't have anything in her name for fear that it'll get frittered away, so she's effectively on permanent financial probation with them. (Again, not my choice of solution, but what can you do.)
Second factor: she's also a disaster with relationships. She either picks truly awful characters, or if she finds a semi-decent one, she goes so overboard with jealous control that she sends them running for the hills. None of her relationships make it past the two year mark and most die a gruesome death before six months. Finally, for good measure, she's increasingly desperate for babies (she has none so far), and time is moving on for her. Back to the present: A couple of months ago she started seeing someone new. He's a complete step-change from her past efforts - solvent, serious, quiet, hard-working, stable - in fact I'm kind of at a loss to figure out (a) why he isn't already taken and (b) what he sees in her. Whatever the case, DH and I think that if she has a shot of making a family happen with anyone, this guy would be a great option, BUT we don't think she's told him about her finances. She swears that she has, but knowing her character as I think I do, I wouldn't put it past her to have brushed it all off as "a little bit of money owed", if she's even said that much, and then rush to get pregnant, because, by her logic, he'd then be stuck with her. We're not sure what to do. Do we pull him to one side and make sure he really does have all the facts? Or do we stay out of it and let it all go however it's going to go? He's a nice guy and I think he deserves to know, but I'm also conscious that it's meddling in someone else's relationship and I'd be furious if someone did that to me. Then again, if she were a serial batterer, I wouldn't hesitate to tell him. I just don't know where the line is drawn here. Also, selfishly, if it all goes wrong and he leaves, it'll end up becoming our problem. She barely gets on with her DPs anymore - no surprise - and she can't even run her own household, so as a single mother there is simply no way she wouldn't end up crashing and burning.
In her favour, I want to say that she is a great auntie to our DCs - they bloody love her, and after a lot of failure and mess, I REALLY want her to finally have some stability and joy and a focus outside of herself in her life. I also think that with the right support around her - and maybe this guy could be it - she would probably grow into a great mother and partner. I would be properly and seriously delighted relieved if he knew the truth, and despite it, they could still live happily ever after, but what's the right way forward? Do we shut up and watch and wait? Or speak with the guy? Or what?? 
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if you read this far!