First, welcome to the board.
Are you wrong for WANTING to do this? There is no right or wrong when it comes to wishes and wants. The real question is "will it be in my best interest to do this?" My guess is that the answer is no.
They have you playing their game. Writing them a letter just makes you another scapegoat, and more deeply entrenches you in the game.
Just because someone hands blame to you does not mean you have to accept it. You can say "I did not do that, and I refuse to be held responsible for it." (the first time you say this, there will be a very long silence and a feeling of glee will well up inside you).
They are chaotic. They have their problems. Do you want to take their problems and make them your own? By attempting to 'fix' the problem, you are actually accepting some responsibility for those problems simply by thinking you have the power to change it. You do not. You can not change them, you didn't cause it, you can't control it, you can't cure it. Only they can do that, and they seem quite happy to not change the current system.
I think your husband is wise to simply avoid them. There is a point where family for the sake of family is nothing more than dosing yourself with poison for no reason.
In 18 years of marriage, can you think of anything good to ever come from his parents? And if the answer to that question is 'no', then do you think one letter is going to change that?
You can not change them. You simply can't. They don't want to change (or they would have by now) and inserting yourself into their issues will only give them another person to castigate.
What you can control is you. You can control whether or not you let them suck you into their black hole of dysfunction or whether you choose to let them ease their consciences with a birthday card here and a christmas card there, and not worry about the rest of it.
From where I sit, I don't think these are people I would *want* to have in my life. I would leave them to themselves to rot in whatever toxic soup they've created for themselves, and I would do my best to insulate my children from it.
Sorry to sound so harsh, I assure you I don't mean this harshly. I guess I'm just not understanding why you would want to keep these people as active parts of your lives when they seem like people you would never associate with if they weren't legally related to you.