I am sorry to hear of your situation.
My viewpoint is that you should allow the family to see her, unless she has stipulated that she doesn't want to see them. By doing that you would be rising above the way they have treated you in the past. I think you should examine your motives as to why you will not allow them to see her, whether it is out of concern for your mother or revenge and a way to get back at them for the way they have treated you. By having your mother living with you for the few years that she has, then surely there should be an expectation that she should see her grandchildren on her deathbed.
I really feel in this most difficult of times that you should put your anonymosity to one side and rise above the way have treated you in the past. At present you are just giving them the justification for the way they have treated you.
If i were on my deathbed i'd want to see my grandchildren.