We went through this with my parents, Ruby. My mother starting losing it first, and my wife, my sister, and I were all so worried about her that we missed the initial signs in my father. After a great deal of family drama (including stealing my father's car because he refused to stop driving after he lost his license) my sister eventually got each one admitted to institutions with memory care — not together, but my mother did not know who my father was, and he kept confusing his in-home caretaker with our mother. No happy ending, alas, just both of them dying peacefully among strangers (which would have been the case even if we had moved in with them).

Re: your cousin's accusations, my mother got highly paranoid before her doctors figured out the right medication, with accusations against anyone and everyone. My father also had paranoid flashes, but we stole his car, so was he really paranoid when we were out to get him? 😉