This all sounds very familiar! My mum's stock response was, 'Well, nobody told me!'
This was her reply when she was told about anything she couldn't remember, e.g:
The doctor came to see you yesterday, your mum died 40-odd years ago, you've been here (in the CH) for more than 2 years, yes, I you do have 2 grandchildren, etc, etc.
It is dementia logic at work. They have no recollection of the event so either it never happened or, 'Nobody told me!'
We got into a strange and unpleasant discussion once when I was trying to explain she had only 2 children, myself and my brother. (She had confabulated 2 sons who lived in America...) She told me that 'everything was different in those days,' (another favourite response to explain the unexplainable). ' Nobody told you when you had a baby - they just gave you one and you just had to get on with it.' Unfortunately she got into this tale and told me that R (my dad) wasn't my father and that she was only a child when she was given her children (she was 29 when she had me!) and you didn't know whose kids they were, etc etc. It wasn't a good day
'Fraid I don't know what to suggest really - it's fairly typical dementia behaviour.
Good luck...