Paranoia and delusions are common and troubling - even cruel. That is because the usual focus of the paranoia is not something exotic and implausible - like the stereotype of the 'x-rays from Martians' that tragically can be seen in young people with schizophrenia . Rather the focus of the paranoia is very often the caregiver who is doing the most for the patient. I sometimes help families by explaining to them that you can understand what the person you care for is thinking when they accuse you of stealing, by considering this. It is frightening and sad for that person with Alzheimer's disease to believe that the people who care for them are stealing from them. However, it likely seems better to the patient, on some level, to think that than for them to think they can no longer remember where they put their money. Fortunately, delusions often respond to treatment with neuroleptic medications.
A sad fact which makes this area complicated is that sometimes the person with dementia is actually being financially abused by their caregivers - who actually are stealing from them. While in my experience this is much less common than delusions, it happens often enough for me to be careful about the information that I hear. Usually, however, delusional beliefs are not confined to just one area, and there are a number of clues in a detailed cognitive interview to give me a reasonable sense of whose story is most likely true. Often, however, a social worker can help sort out exact details, and I have been wrong often enough to be leery about just listening to one side of the story.