Down Syndrome and Early-Onset Alzheimer’s often coexist because of a genetic abnormality, which is pretty awful. So, the seizure medication COULD be causing this, but, more likely, this is just a part of her dementia. What you all need to do is this: see those people, too. Let her know that you’ll do your best to keep her stuff safe. Telling her that she’s wrong, and that people aren’t stealing her stuff, is just making her more mad. You can’t completely solve the problem, but you can alleviate some of the pain.