Bad News and Dementia

Is there any way to tell a person with dementia that someone has died?
I got a phone call last night that Mom's favorite cousin, Walter Pera, had died on July 4 at the age of 92.
Today I went to visit Mom and tried to gently give her this news. She responded well and appropriately: "Oh, I'm so sorry to hear that. I loved Walter. He used to ask me to dance at school dances when I had no partner. He taught me to play tennis. He and Reynold caught a red fox and tied it to the clothesline. It ran up and down."
"Yes, Walter was wonderful. He was 92 years old. Reynold only made it to 87," I commented.
"What? Reynold died? You didn't tell me that Reynold died," she cried.
"Yes, he died two years ago. I told you but you don't remember because we didn't go to a memorial service. He didn't want one," I said.
But the damage was done. She was grieving all over again, deeply, for her brother. The news of Walter's death was not difficult for her, but re-entering her grief over her brother caused her to become agitated.
I shouldn't have told her about Walter.
Memo: Don't share news of deaths again. It is confusing for her and painful.