Mom of 3 - You're not going to win this - been there and understand totally. So, my advice is to "agree" to everything your son wants - you don't count and frankly, if you stand up to your son, your DIL and her parents, this will turn it against you. I know - as I have said - been there. That fact you are grieving your husband will have no influence on your DIL or her ILs (sounds as if they are extremely self-centered to the point of having mental health issues); sound cruel - but they really don't care. It's all about them and if your son wants to remain married; then he will be required to conform.
Your saving grace will be your "niceness" and "not wanting anything in return" and acting "graciously" (because frankly, you're not going to get anything). But someday, if your son is like mine, it will end (for me it was 15 years), and then you don't want to have to deal with past "baggage". I so wish (like ccarern) that I had said or done nothing. I question the use of a counsellor - I know our son would have told his wife and it would have backfired on me (and there is no way that she would ever be in the wrong). If you use a counsellor, please go alone at first and see what the counsellor thinks would work. I have been accused of things I did not say, do, or even think but as a MIL, almost everyone (if not everyone) will blame you. Please tread very carefully; your son does not yet empathize/sympathize with you and as I said, you won't win. One thing that did help was the constant intervention of our younger son and his friends who could see the injustice of it all. And trust that those years of loving upbringing will someday be remembered. But, the wife rules and as long as that marriage is intact, you won't/can't win. It will get worse after the grandchildren arrive, so please, be careful.
As many will say on this forum, continue with your life - putting your son "on hold". This is the only way I see that you will be able to keep him in your life. When our son finally did divorce his wife, he gave all holidays (even Father's Day) to the ex-wife and her family. It gets better, but it never gets great. As a friend said to me at church today - "it depends on the girl who becomes the wife". Be kind and gracious and thankful for any crumbs thrown your way (there won't be many). One of our grandchildren is now beginning to rebel against the maternal side; I don't know if we'll ever "know" the other grandchild. Now after saying all this about our elder son, our younger son married the most wonderful girl in the world. Is there any in-between? So sorry ....