My heart is breaking for you. We went through this last year with my "mother" in law. Please, do not beat yourself up or feel guilty about anything you didn't do.
I'm sure you have been a wonderful daughter. How could you have known anything was wrong, it happened so fast. Be with her as much as possible, as you know, every moment is even more precious now. I pray that God gives you strength to get through this. I also pray that she doesen't have to endure any pain or suffering. Your mom is very lucky to have you with her. I only know you from on here but if you need ANYTHING, please pm me.