That is a tought place. I'm fortunate that my parents respect my position, even if they don't agree with it.
When my son was younger, I told them that I had terrible memories of staring in horrified fascination at those bloody gory pictures and that it was not ok to read it to him. I provided other books that I approved of and made it clear that he was NOT being raised as a witness.
With my parents, I never had to draw any more line than letting them know that I am the parent and it is up to me to decide what my children look at. I know that is rare.
I hope you find a workable solution and I understand your statement about wanting your Mom back. I feel a hole every holiday season because I know that prior to my mom becoming a witness, she loved christmas and I would love to see her share that experience with her grandchildren.