I agree with so much that has been written here. I kicked twunt out after 22 years of marriage, and it was a total shock to my parents, family and friends. Only our kids knew how unhappy things were, because they lived in an unhappy house. And yes, Twunt did the whole script thing too, denied an affair (he's now living with her), distanced himself from me and the kids, re-wrote history to make me out to be totally to blame. I didn't tell my parents, we were going through an awfully sad time as a family, they were suffering terribly, and I didn't want to add to their pain and burden. It is possible for people to think that you are happily married, when you're not. After the event, so many people said that things now added up, made sense...but at the time we gave an allusion of happiness, and people took it on board.
Two things that still upset me: firstly, that twunt, never, ever, told me how deeply unhappy he was before things got to the stage where an affair (because he didn't want to be alone, so I totally get the monkey analogy upthread) and leaving was the only way out. I'm not saying the outcome would have been different, but I felt that after being with him for 24 years, and having 4 kids, our marriage had the right to be attempted to be fixed. He really did convince me that everything was ok, and would not discuss any thing that would help us - relate, changing our lifestyles, making time for each other, were all dismissed by him, because he said "everything's ok". I knew it wasn't, but his point blank refusal to co-operate still rankles. Then he said he was off, and there was no discussion, he simply ignored and walked away from all my attempts to discuss things. I still, to this day, don't really know why and I need to know, but am learning to accept that I won't.
The second thing is that my family, in particular my parents, took him into their huge arms and welcomed and loved him from the first day they met him. My parents treated him like the son they never had. I told them I'd kicked him out, and they haven't seen him, talked to him, or discussed anything with him since then. They are deeply hurt, not only by the pain he caused all of us, but the pain he caused them.
Ronan Keating said on Piers Morgan last night that he deeply regretted the hurt he had caused so many people. The devastation, he called it. And it is devasting, the aftermath of a marriage break-up, and the ripples hurt more than the immediate family.
So I feel for you OP, but all you can do is be there for all concerned, and try not to take sides. Their marriage was theirs, and theirs alone; no one can tell them what to do. But he's your brother, so you have to decide how far you can support him. You don't have to do more than you want too. As an aunt, you can be strong and supportive and a rock to their kids and their mum - that would be wonderful.
Saffysmum x