Can your response focus on how much you love all the parties involved and how important they are to you? You are sorry for disagreements, you thank them for their insights, you realize that people inevitably have different points of view. Most important: you have seen so many families torn apart by trivial differences and you are thankful that she, grandma, is not the type of person who would let that happen.

Then move on to something current and positive and simply refuse to be sucked into the argument.

Good luck. These things are always difficult and, I think, sometimes hopeless.