I have a son that has been diagnosed with major depression and other issues. My sister which I think she means well, but she is convinced that he has a demon in him. How do you even talk to a person that keeps trying to make you and your child believe this?
Many people don't really listen. They only hear what they want to hear. Your sister is convinced that demons still possess people, despite Zechariah prophesying that when Christ brought salvation to the world, that demons would be driven out and prophesy would end (Zechariah 13:1-3). See: What About Demon Possession Today? Unfortunately, no amount of reasoned arguments will convince your sister otherwise. You will have to either ignore her comments or politely tell her that you no longer want to hear her idle speculations.