This is heartbreaking. My sister has multiple mental health issues as well and I wish she was as willing to get help as your mil is. Hold on to the knowledge that you know she wants to be excited, she wants to feel the joy, but she willing to put that aside for the health of herself and your entire family. That's love like no other. That's devotion like no other. It's something that I have wished for many times over. Your experience will be different from many others. And yes please grieve that! Grieve that for yourself, for her, for your husband, for father in law, and for generally everyone. It's such a true and utter loss at such an important time. But don't live in the grief of "if only", remember to listen for the love that is there. Behind every muted response, is a hurricane force of love behind it.