Interfere seems a bit strong but felt it might garner more traffic (sorry) - what I really mean is should I talk to my BIL before my DSis calls time on their marriage for good?
The only reason I suggest this is because him and I get on very well and I think he might listen if he hears it from another source?
Basically, she's unhappy and he's not listening. It's like he's in denial. She's in the midst of an affair (approx 6mths now) he knows about it but is turning a blind eye, I think, in the vain hope that it'll fizzle out and they'll all go back to normal. She's taking it that he just doesn't care. When the affair started (one night stand), he found out, begged her not to leave, promise it was just a one night stand and he would forget about it. She couldn't as was crippled by guilt so asked him to go to Relate with her to get to the bottom of why it happened in the first place. He reluctantly dragged himself along but as he said he forgave her and wanted to move on then he felt the process to be pointless and money they didn't need to spend. 
Therefore, the affair started up but with intent on both sides this time.
He's like an old man before his time (they're the same age, mid 30's and married for 4 years). She's constantly trying to get them to do stuff together, even stuff out with her interest such as going to see his fave football team but his response is 'I can watch it on Sky, what's the point?'. It's not a money issue as they can afford to do it.
Anyway, we spent several hours on the phone last night and she's a mess (I'm not condoning her affair, she knows this but this guy has given her a window to how life might be - problem is, he's also married and there's kids involved there. Not good)
To go back to my BIL, I'm wondering if I called him and said, 'look, you need to pull your head out of the sand, she loves you as a person but life is too short to be miserable and one day she'll pack up and go with no coming back, if you don't want that, then you guys have to do some serious work and soul searching to get this marriage back on track'
One bit of me says, not my business. One bit of me says, I love them very much and he's a good man (albeit needing a kick up the arse)
If my DSis found out, it wouldn't cause a terminal fall out between us but when I suggested a while ago that I speak to BIL she said no as 'he needed to understand himself'- the guy is clearly struggling though.
I'm married myself and consider myself to have a very good marriage but I know that takes work and it's not all magically working with no effort on both of our parts.
Go on, flame me, tell me I'm a nosey busybody or whatever. I'll just be sad if a marriage ends when all avenues weren't exhausted.