How to respond to "why don't I remember that?"

my DH will ask the same type of questions. After a recent visit we had, he asked that question, I said “probably because it wasn’t that memorable” and laugh. Usually that’s all he needs and he doesn’t go into any more detailed questions. I have shown him pictures like you, but he never seems interested in looking at them or will say something negative. I just go by what seems to work for me and my DH. What’s hard for me is that there is so much that I don’t even bother conversing about any more because I’m never sure of the reaction or non reaction I might get.

Since you've gone down this road before, you might skip the process and when he asks when she's coming just say, "I don't know when she'll get out here again." If he reminds you that she was going to come you can comment about how hard it is to plan anything these days.