Hi RosieF
and a warm welcome from me too
I wonder - if you listen to your mum's conversations with your brother, is she actually giving specific and accurate information? My dad was very good at covering up with general 'polite conversation' type comments and answering questions in a vague way - to the other person it seemed as though he was really taking part in the conversation but for me, listening in, it was clear that dad was not talking about the lunch he had just had eg tomato soup and ham sandwiches, when he said it was delicious and he'd eaten lots and was very full now, and yes of course he'd enjoyed the pudding ..... Dad had a lot of stock phrases he used to cover up that he didn't really know what to say.
We've all been trained to 'chat' about the weather and general stuff, and the 'hostess mode' kicks in using these clichés - but it's just telling the other person what they want to hear, not actually passing on real information. (Not surprising as we all do it = 'How are you?' 'I'm fine, thanks for asking' when actually you are dog tired and fighting off the flu)
With you, your mum can relax and isn't putting on a front - so you get the real confusion and forgetfulness.
This 'hostess mode' can actually be a real nuisance as many people don't listen carefully and are fooled by it so don't believe the person with dementia is 'as bad' as the carer knows s/he is. I hope your brother does realise that your mum is effectively putting on a show for him.