My Nan Doesn't Know Her Son Is In Prison - From Anonymous

Hello. Please keep my name anonymous. My Nan is in a dementia home and my uncle is in prison.
When I go and visit my Nan, she mentions my Uncles name and asks when he is going to visit her. I don't know what to tell her because she has dementia and I don't want to upset her by telling her he is in prison. Sometimes my Nan remembers things and sometimes she doesn't but that is because of her illness. She is in the early stages of dementia so does remember names sometimes.
My Nan won't see my Uncle which is her son again because he is in prison for a very long time so she will probably pass away without ever seeing him again unless the prison allows him to see her when she is in her last days but I don't know how it all works. Lots of love from Anonymous.