Do I tell my mom that my brother will be going to prison?

My brother committed a crime, pleaded guilty and will be sent to prison this coming week. We told my mom that he had done this when it happened 8 months ago. She occasionally mentioned it, but hasn't for the last two or three months. Her memory is very bad and I know she won't remember more than a day or two if that long. He has cut off communication with her now. He doesn't/ won't talk about why. Do I tell her the truth, or just continue to say he's fishing/golfing/or whatever? To keep telling her will upset her over and over and she'll ask the same questions. I hate to not tell her the truth, but is telling her the right thing to do?