This thread has been validating for me because our raised in son (since he was 5) and my husband and I are experiencing the same thing. I think there has been really good advice here and we have been applying much of it. Our son did do some venting but not nearly as bad as the letter you received. I knew he needed to do that. We are giving them space. His wife too, I feel, has much control, along with her family. However, she's not really a hostile person and we are hoping for the best. It's going okay, the worst is over.
I hope Totally that things turn around for you too. Give it time. There is some truth too to what the Arabian said.