I had a horrible relationship with my mother all my life, but at the end of hers, she forgot about it all because of the increasing dementia. We still had some conflicts when I went to see her, but I let them slide. I figured, WTF, she's 96 and either doesn't remember or has completely revised our history, making any recitation of it pointless. I don't know how cognizant your mother is, but perhaps you should do the same.

Ask yourself if it would have done any good or helped the situation.

Hey, she's 90 years ol...
I'd let it go, if you can..

I would have said "Thank you for the reminder, Mother. I will consider what you have said." and then let it go. It upsets you, she will never change or admit to wrong doing. Don't let her get a rise out of you, be as non committal and non confrontational as possible. Remember, they can't hurt you physically any more, and you can minimize the emotional hurt by disengaging. I am sorry you had such a crappy childhood, however you're stronger for it.