99th birthday is awesome!! No one on either side of my family has had that longevity (yet) so it's hard for me to even guess what might or might not work. As someone on this board said "every stroke is like a snowflake."
With my mom, her long term memory is great, everything after the stroke is mush. One thing I have to do with my mom is not surprise her with anything! I have to start days ahead of time telling her what is going to happen, give her choices, and repeat it over several times. So before she called her Aunt I told her "Mom, you need to call Anna, her birthday is this week, do you want to call her today or tomorrow." She said tomorrow for about a week before she called and the conversation went beautifully, or so I was told...(it was in German and I don't speak German.) LOL
Maybe you could ask her "mom, its your brothers birthday, we should call him...what are you two old birds going to talk about?" Get her to think about the conversation before the call. I am slowly learning with this stroke stuff that I am never, ever going to have the right answer, but it's not going to be completely wrong either.
PS. For my mom's therapies, I bought a white board to write down who is coming what day / time and to track her appointments. I would remind her the day before what will be happening the next day. Time has helped and she doesn't rely on the board anymore, but I still tell her the day before what is happening the next day.