I’m so sorry you’re going through this, HoneyBeeCee. Unfortunately, wild accusations are not uncommon once dementia has reared its ugly head.
My mother became furiously convinced that her sister had ‘stolen’ their mother’s house, i.e. their childhood home. It was all entirely in her head and I swear sure that even a signed and sealed declaration from the Lord Chancellor wouldn’t have convinced her - he’d have been ‘in league with’ her sister, like anyone else who tried to put her straight.*
Now and then she also made horrible accusations against my dh and dds, that e.g. they ‘didn’t allow’ me to visit her, or tried to prevent me. It was very upsetting - and nothing could have been further from the truth.
*in the end, since it was useless trying to convince her, I used to say e.g. ‘Dear me, that’s terrible, I had no idea! I’ll get on to the police/a solicitor first thing tomorrow.’
Her short term memory was practically zero by then, so I could repeat this ad lib - and I needed to!