She sounds like a very self centred lady, she has drink problems and seems to have no appreciation for your son or you. If you were to DA she would use it as the excuse to have nothing to do with you. We invest a lot in family relationships and sometimes try longer and harder than we should because of the blood connection.
She is mean because she probably has mental and alcohol issues that mean that she sees things constantly from her perspective. The faith she belongs to also makes her judgemental and gives her permission to be unpleasant and a framework to judge you.
She is not a healthy person to be around at the moment. When you are in her company be firm about what you will and will not accept in terms of your son. If she accuses him of being naughty then I would be direct, "He is not naughty he is autistic as his grandmaother I would hope to see a little more tolerance and understanding."
You have nothing to lose.