Hi Sarah
I am so sorry that your mum cannot comprehend what is happening to your dad, I had the same problem with my mum to begin with. I also had the same problem with my dad when his twin brother had dementia. I found that both of them dismissed any explanation that I gave. However, they were both prepared to listen to professionals, GP, social services, occupational therapy, mental health. I do hope that the opportunity arises for mum to talk to professionals, and can come to terms with it a little at least.
I'm sure that others will be along to give you better help and hope the link may help too.