Noticed behaviour of a family member). what causes an inability to read or analyse human behaviour and conversational cues? along with paranoia and a highly suspicious nature.

Many many things. Go to uptodate.clans read more. Then keep diary documenting issues and share with persons medical team.

Some thoughts and perhaps advice: there maybe a personality disorder such as borderline personality, but there could also be a thought disorder, amongst many possible disorders. For your peace of mind, visit with a psychologist and get some clues how to deal with this and what risk factors to react to, as the full dimensions may be challenging.