Hi, I'd like to help you with your question.
It sounds like your mom is going through a normal process of dealing with what happened. It is painful but necessary so she can heal.
Any loss will cause grief and overwhelming feelings. Your mother's grief is compounded by the feeling that this was her fault. So in a sense, she has twice the amount of feelings to work through in order to feel better.
There are some steps you can take to help your mom through this:
One, let her know that feeling the grief and guilt is ok. She needs to allow herself to feel them, even though it feels so awful and is hard to do. If she tries to avoid feeling these feelings, she will only delay the inevitable. The idea is to work through the grief, not around it. It is the only path to healing.
Two, be there for her. Let her know she can call you or talk to you anytime she needs to. It is not going to be easy for you, either. Be sure you have someone to talk over your feelings with as well.
Three, suggest your mom write a letter to your grandmother. Have her tell your grandmother all about how she feels. Don't hold back. Take as long as she needs to write it.
Four, suggest your mom seek therapy. She is trying to cope with two overwhelming emotions. She needs someone who can tell her what is normal to feel. And she also needs someone to talk to besides family and friends. She can talk to her doctor for a referral or if she attends church, her pastor can help. She can also search on line at http://therapists.psychologytoday.com/rms/.
Five, find a support group either in person or on line. Here is a link to help: http://www.onlinegriefsupport.com/.
Six, educate yourself and your mother about grief and guilt. Here are some resources to help:
Survivor Guilt by Aphrodite Matsakis
Understanding Grief: Helping Yourself Heal by Alan Wolfelt
How To Go On Living When Someone You Love Dies by Therese A. Rando
You can find these books on Amazon.com or your local library may have them for you.
Also, if your mother does attend church, she may want to consider talking with her pastor or a counselor at the church. Faith can make a big difference in understanding why things happen the way they do and how to move forward and feel better.
I hope this has helped you,