Yes remember your first thread.
You wouldn't be unreasonable to send it yourself... because this is an issue which very much affects YOU. And you want, quite reasonably, to get it sorted.
Sit down with your DH and point out that SIL is what she is. Him ignoring the situation (or trying to) or (worse still) writing letters as a sop to you and then staying firmly on the fence won't make the situation better, it will make it worse because SIL will still be mad and unpleasant but in addition to that, YOU will start to lose respect for him and start to believe that he puts her peace of mind above yours. And that is a dangerous position for him to try and take.
Point out that how to deal with this situation has been much discussed, and a solution has been found. If he does not want to stick to what has been decided, and to send the letter (or any sort of communication which tells her where to go), then that is his decision, but a letter will still be sent, because YOU will send one. Point out that it is your right to do so - she has been and is badly affecting your life, and potentially that of your DD. He isn't in control of whether a letter gets sent and whether your DD is kept away from her or not. That's the bottom line.
Of course, it would be better if it came from him. That would have a double benefit:
-it would have more credence in SIL's/MIL's eyes and give less space for comeback;
-it would reassure you and DD that he is on the side of your own family and does not support her unreasonable behaviour.
So you could say, choose. If you don't send a letter, I will - and it will have to be quite a different sort of letter - because basically it will have to cut contact with BOTH of them, PERMANENTLY. Because if the letter comes from you alone, it will merely inflame SIL - 'You're taking my brudder away!' - and you refuse to deal with that. So you will have to state that you no longer wish for either of them to be in your lives. Needless to say, you then assume that SIL will ramp up the manipulation tenfold, and that will ALL be on his plate, because you will have NOTHING FURTHER TO DO WITH IT. So if he wants to pile on his own stress - simultaneously inviting an onslaught from SIL, as well as completely undermining his own marriage - be your guest. You'll at least know where you stand when it comes to him supporting you and DD, when the chips are down.
If a letter comes from him, it has more strength in their eyes, and (as discussed) allows for the possibility that MIL can still be a part of your lives. That is of course the much nicer and less stressful option.
I know what I would decide if I were him. Shame on him, actually - and silly man - acting like this really isn't going to make this problem disappear.
Your main aim I assume is to protect your DD from her craziness, so if you keep that in mind you can't go far wrong. Thank God for distance eh?!