He said he's seeing someone new

[4ever_Regretful]

I enjoy novels, so you just keeping writing (typing).

I hope you enjoy the production! Positive things can still happen w/o your H.

So now my brother and sister are tempted to write him a letter in a similar vein. Who knows what it would end up saying if they ever do get around to writing it, though I imagine parts of it may not be that nice, considering my sister expressed last night that if she ever were face to face with H's new girlfriend, she'd beat the crap out of her (my sister isn't a violent type) and tell her how disgraceful she was for hooking up with a married man.

I think the letter is a great idea, but content is important.
If it comes across as being sincerely concern for his well being, I say go for it. Perhaps this will also remind him he is still family and he is not just breaking up a M, but a family.

Part of it may also be that he's trying to mess things up even worse in an effort to convince himself that there really is no going back. The bigger the mess, the harder it'll be to fix, so the easier it is for him to believe that his new, carefree life is the much better option.

I agree with your observation. And the comment about the letter acting as a wake up call for how serious he is may just prove he is taking action to prove to himself as well.