Mum is constantly talking to herself. Any advice?

Hi,

My mum is 82 yrs young.

We lost her son, my brother, 2 years ago and mum has been pretty fragile ever since.

Mum spends most of her time at my place I constantly find her talking to someone, I presume it’s my brother who passed and it’s starting to frighten me.

I’ve spoken to nurses they say not to worry about it. Mum doesn’t have dementia but could this be a first sign of it.

Sorry for my rambling.