oh Andrea, I'm so sorry. you know we are always here to listen and talk.
Everyone deals with grief in different ways, as you say, your SIL shows no attachment to her loss and that's what works for her, but it's not for everyone.
I think we would all agree that what your MIL said was completely out of order, and the fact that your 11 year old son recognised this says it all.
I'm like you - if someone in my life can't understand why I was upset yesterday (and every other day), then I don't want them in my life, end of story. Life's too short to be pulled down by people like this, no matter who they are.
you do what YOU need to do, for you, your OH and your boys, and ignore everyone else.
xxxxx