Thank you SO much for sharing that, Bear. I was tearing up while reading it. Your situation is almost identical to mine. It must have been a HUGE moment when your aunt finally opened up to you. I really hope that I get to have such a moment.
The things you wrote about your grandmother also hit home. I know that my grandmother knew more than she ever let on. She, too, seemed to be walking on eggshells. As she got older, she finally started to let things slip out. She lived in Canada, so I didn't get to see her in person very often, but I wrote to her constantly. She NEVER said anything bad about my family in her letters, but near the end, her tone began to change. She stopped referring to my brother (the GC) by name, and began referring to him as "the spoiled brat." Instead of referring to my mother as "Mum," she became "your mother." Very subtle changes, but I think as she came to the end of her life, she stopped caring about being politically correct, and quit worrying about the wrath of M.
My grandmother also knew, somehow, that M had been refusing to congratulate me on getting my college degree. Two days before she died, a cousin who had been caring for GM at her death bed, emailed me and said that Grandma wanted me to know how proud she was that I had worked my way through college, and that SOMEONE needed to tell me. I think that if I had flown to Canada to see her when she was dying, that she might have opened up about it. But she found other ways to get the message through. She definitely knew, and I think that like your grandmother, she was probably also afraid of what M would do to her if she didn't keep quiet.