Not so long ago I had a somewhat similar family situation. As a long time Buddhist I tried to not become angry, but the situation became so dire I had to act. The action came in the form of a court battle, and I did my best to be outwardly civil, although internally I was less successful at rising above the matter. To make a long story short, I won. It was only after
I was able to stop the elder abuse that I was able to begin to enact some teachings. First; to not seek revenge, and second; begin to forgive.
I am afraid of taking the wrong action and making things worse, or taking no action and making things worse...
There is a Dharma imperative to repay the kindness of one's parents. In this case it would be the kindness of one's grandparent for having raised you. Whatever you can do that is motivated by love for her, with the least amount of selfish and self-righteous motivation, is probably the right thing to do. Maybe it is nothing, that creating conflict will be worse for her than doing nothing. Plus there is the issue that she is an adult, and has choice. Only when people cannot protect themselves should we appoint ourselves their protectors. With children this issue is clear, but with adults there is a very wide gray line in this regard. Only you can decide what to do or not do.
The Milarepa story comes to mind, where Mila was not able to save the situation but took revenge after the fact. He suffered greatly because of the negative karma from his actions. Whatever action you take or don't take, don't be vindictive, and do try to not condemn, but reserve a space in your heart for forgiveness, if not for now then for in the future.
This is what I learned from how my karma played out. I know others could have done better in my situation, and if I were more spiritually developed I could have done better, but at the time that was the best I could do. Doing your best is the most you can ever do, even if in the end it doesn't work. Good luck to you.
Unless I am quoting scripture or a recognized authority my posts are my opinions only and not "Dharma". I offer my posts for entertainment purposes only. I hope you find them either interesting or enjoyable--but don't take anything I say to heart without running it by your own teacher first!