This whole situation is sad. She needs help. I am quite sorry you are going through this.
As a mother of adult children I crave grandchildren. But, I keep my feelings to myself. This IS, afterall, their decision to make. We have not talked about it - I would never put them in the position of them thinking I feel they are lacking or disappointing me. At this point, I doubt it is going to happen. They are 32 and 27 with burgeoning careers. I just don't see slowing down in their plans. And that is what is important to me; that, if they do have children that they be planned.
I am unsure of how to deal with your mother other than to put your foot down and explain giving her grandshildren is a matter only of your choice and that it isn't in the cards. Perhaps she has a social or support network to talk to someone about her hopes and, quite obviously, her fears. What she is doing is unreasonable. If she won't listen to this reason you may just have to limit your ties with her such that they be on your terms only.