Will Mum remember if her son is in prison?

My Mum has just gone in to a care home. 
Her dementia has got worse and sadly it has ended this way.
My brother (her son) has just been sent to prison and my Mum is calling out his name all the time asking where he is.
My Mum's health (dementia) has declined quite quickly and our family do not know what to tell her about my brother. If we told her would she remember and be tortured by it or would she forget and continue calling out his name? There are no answers but we want to make this as less painful as we can if this is at all possible. 
Obviously Mum isn't going to see her son again. That is the harsh truth because his sentence exceeds Mum's life expectancy. It is heartbreaking! 
The prison won't allow my brother to visit Mum in the care home so we are at a loss on what to do. When all said and done; he is her son and all this is a nightmare.
It is family issues as such that people do not recognise when someone goes to prison. Prisoners need to sit back and think about the train wreck they leave their loved ones because it is us that always have to clean up. 
I am furious with my brother but life is literally too short to vent the anger. I need a solution but the light at the end of the tunnel does not seem to be working right now. Any advice? Anyone? Love from YVH