I think this is very difficult to answer, since the poor woman must be scared enough as it is, without her suddenly finding that she has far less time left than she first thought.
I knew someone who had the same disease, and in fact, he lived for far longer than was expected, so imagine if that happened to your mil. After the 6-8 months, every day'd be full of wondering if it was going to be her last.
If it was me, I think I'd put it to her that these things can suddenly accelerate, and that if she has anything that she'd like to do before the worst happens, then maybe she'd like you and the family to help her with them - e.g. arrange for her to meet up with friends, maybe go out to places, etc. It's sad news though, and I wish you all the best.