I'd just like to add a point to Ms. Koffend's excellent remarks. Please try to keep in mind that your mother-in-law is in a "sandwich" situation here that she probably never envisioned, or if she did, it was her worst possible nightmare. One can intellectually accept the idea of one's parent dying, especially if s/he has been dealing with her parent's illness for some time. If I'm remembering my college psychology course right, dealing with the death of a child is just about the highest stressor we humans can experience.
Your m-i-l needs to have a sense of control, that she's doing SOMETHING for her child. If there is something you can delegate to her, that she'd be in charge of UNDER YOUR GUIDANCE, it might give her that sense AND relieve you of one more task in your increasing burden. Maybe she can research treatment options or run errands or cook meals.
Another thought: If your husband has siblings, you might want to speak to them about this as well. They would have many more years of experience dealing with your m-i-l and might have some good suggestions.
In prayer that your worst fears are unfounded,