Granddaughter verbally abuses grandmother?

The way I see it, you have two options. Be there to support her and be her shoulder to cry on, or decide that this is a deal-breaker for you, and remove yourself from the situation. This may sound harsh, but I'm coming from a family with a similar background. This decisions to be walked over are Grandma's. If she decides that she won't take it, you can be there to help support her in that decision, but if you try to step in, you will most likely be resented by Grandma for not trusting her to make her own decisions about trying to help her grandchildren by giving them a safe place to be. I know it is frustrating and heart wrenching to watch someone you love go through something like this. Good luck and thank you for wanting to make her happy.