Complicated...remember you posting before.
So your mother (maternal grandmother) is an enabler/bystander to your dad's (maternal grandmothers 2nd husband) a bully ie emotionally abusive/neglectful.
Your mother says (maternal grandmothers 1st husband, aunt and biological mothers father) 1st husband hit her and was terrible to your mother (his daughter) when she had you adopted.
From my perspective, I have broken contact with my parents...so with my dd I intend too:
Tell her why, but not in a way to make them out to be criminals! Just in a 'they were not okay with me, but your relationship with them is up to you and them'.
To allow her contact when she is emotionally mature enough
To try not to let my feelings interfere with any relationship she has with them. If she gets something good out of it that is good for her.
If there were a physical threat to her being in contact, I would wait until she was 16 and explain why.
I think your mother (maternal grandmother) is being unreasonable. Yes she has strong feelings, however if you want to have some kind of relationship with her ex husband, your grandad, that is up to you!
I do think it important to reassure her that you do believe her experience of being married to him. However he is your grandfather and you would like to have contact with him for that reason, if he should ever treat you the way he did her, that would be the end of things. But you want to try.
Hope some of that helps...just don't let your mother dictate who you get to know.