They clearly aren't interested in dealing with conflict in a positive way. This is a lesson you've learned about your family.

There is an old saying, 'the tallest nail gets pounded down first.' Often among families or other close long-term groups people prefer to pretend that bad things in their past never really happened, because it is easier to never confront them. Your confronting of your grandparents wouldn't let them live that illusion, you stood up, and now they are using gossip and second-degree insults to knock you back down because it is easier to keep everyone in line than it is to resolve the hurts and conflicts of decades.

Maybe your aunt is so critical of your letter because her parents treated her in much the same way they treated you? Your letter could have forced her to relive her own childhood, and made her question her own choices. This is something very uncomfortable for most people. Maybe she wishes she had stood up to them, but now feels so very in debt due to their financing her lifestyle that is she is trapped? By confronting your grandparents you may have inadvertently have caused your aunt to confront her past self and realize she doesn't really like the person she's become. When that happens, a person might grow into someone that looks more like who they'd want to be, but they might also decide that their current state is outside of their control, and lash out at the person who made them doubt themselves.

She is thinking to herself "I put up with their abuse for forty years, and I never made a big fuss about it." To her, you standing up for yourself can only be disrespectful, because some weird interpretation of the concept of respect is the only way she can justify herself to herself in voluntarily taking their abuse for so long.

People often put more effort into justifying their past than they do growing into their future. These are people who choose to let their past be their future. It doesn't have to be. Don't be that person.