mintyy please just stop? You don't support the op, you have nothing constructive to say and are picking at everything anyone with experience of matters like this have to say. Your question to me about relevance just illuminates how out of your area of competence you are here.
It's good that you don't have the first clue about such things. Seriously. Please don't attach your experience or lack of it to those with direct and real experience of what is a horrific and painful situation.
This is a situation where whatever you do, you can't win.
Send a letter in reply? They won't hear anything that's said about their actions being hurtful. Furthermore it feeds the narcissistic need to drag the victims back in, opening a can of worms that can't be resealed. The letter can also be used as 'proof' of how awful op and her dh are. "SEE? After all we've done for them..." And on goes the drama, dragging other winged monkeys in to the mess that they are thriving off. One reply can encourage more nasty letters. Those letters are designed to invade the op and dh life. The first - I think - was sent 'signed for'. That in itself is against the very nature of an olive branch.
Or no reply? With this situation it's a case of refusing to get dragged in, no matter what's hurled. It's VERY hard to remain disengaged because these people know exactly what buttons to press. If those buttons don't work, they'll press more.
If any one of us had a falling out with our children, we'd move heaven and earth to find out why, we'd apologise first and ask what it is we'd done.
There has been none of this. Even in isolation, these 3 letters have not once apologised or made any offer of conciliation that hasn't been a veiled threat of an alternative scenario happening if they don't 'come to heel' and fast.
The mil here told the op that there would be no contact. And no contact it is. The relief of the dh is so significant here. Please all see this?
Imagine what you would have to endure to finally arrive at that sentiment with people you have known your whole life?
Step back and empathise what it would take for you to HAVE TO take that decision.
If you really genuinely can't, then please perhaps admit defeat on a subject rather than force your blessed lack of experience upon someone who is in such an awful situation and can do no more other than to go Nc for their own sanity/safety/wellbeing.
we might not like the decisions of other adults, but we can't change them. We can only make decisions for ourselves as individuals. That applies to both sides here.