@ShakespearesBlister - I'm sorry, but you're making a whole lot of wild assumptions about the wife in that statement. Deranged, deluded... there is absolutely no evidence to support such extreme claims.
The wife and the other son are a victim of this jerk, too. Obviously we have no idea what she was thinking or what she knows... but it's entirely possible that she doesn't KNOW about the affair, the child, or the letter that the husband sent.
Just because he TOLD the OP that he told his wife the truth, they decided to stay together and pretend she and her twins do not exist, and that he has her full support on his cowardice doesn't mean that he didn't just fake her signature on the letter!
Anyone can write a letter and claim it's from multiple people, but the reality is that the other people know nothing at all about it!
BUT... if she actually does know the full story and approves of her husband ditching his own twin children with another woman, then that would make her a jerk, yes.
But the claim that she is 'deranged' is way out of line. For all we know, they had an open marriage, but the only condition was that he would not get any of the women pregnant, or if he DID, would not have contact with them!
Who really know what she knows, thinks or wants?