I do not believe my aunt has personality disorder so maybe you should see a therapist but if you still feel like it's a demon I would suggest you burn some stage and wear a crystal or put crystals around your house
Possession is a very, very rare occurrence. The likelihood of possession being the problem is slim to none, especially if the person does not practice any form of invocation. What is most likely the case is a personality disorder. Personality disorders can mimic 'possessions' in the sense that they can heavily change how a person thinks or reacts to situations. These can be both big and small changes.
However, strictly on the question of IF there are beings that can change a person, the answer is yes. A powerful wight or daemon can give off an aura of energy strong enough to effect the energy of a person in the same way another person can. If one spends a great deal of time around a negative person, one can adopt a negative personality. This is also the case with spirits (wights, daemons). They have the capability to change a person's outlook with their energy and through effecting the person's life in positive or negative ways.
Again, possession is extremely rare, so if you are looking for answers to an occurrence of this happening, it is more than likely a personality disorder.