Is there a name for a person that believes that whatever they think up in their heads must be the absolute truth? I have a sister-in-law that keeps harassing my husband over things that she just dreams up.
If there is no underlying personality disorder then she is just either delusional or has an over active imagination depending on what she is dreaming about. However you might just not like her and THINK she is believing things in her head. Or your husband is playing about. If there is an underlying disorder, then there are loads of types of delusional disorders and paranoid disorders depending on what this sister in law believes in her head. It might be that you just don't like her.