My grandma told me something about my brother that makes absolutely no sense.

Throwaway account for this. My grandma and I got talking about my brother (48) who is disabled. She said she watched him eat food then fell face first onto the floor dead in front of her. Now I know it sounds like a weird dream but here is the thing. I don't have a brother. I had a sister who died in 1996, but I do not have nor ever had a brother. We both verified this. I have two sisters now and a deceased sister but no brother ever. Also my memory of my sister is solid. I have never once thought I had a brother. I know who my brother in laws are and who my brother from another mother is. And in no universe ever have I had a brother. My grandma's age is bothering me as most people with cognitive issues. She insists that she knows what she saw and is not confused. what do you think the explanation is?